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Informal idea of a limit

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51011212
Evaluate \(\lim_{x\to\infty}\frac{x^2+4x}{2x^2+3}\).

Hints

- Divide by the highest power of \(x\) in the denominator. - Determine what happens to terms with \(x\) in the denominator as \(x\to\infty\).

Solution

1. Divide the numerator and denominator by \(x^2\): \(\frac{1+\frac{4}{x}}{2+\frac{3}{x^2}}\). 2. As \(x\to\infty\), both \(\frac{4}{x}\) and \(\frac{3}{x^2}\) approach \(0\). 3. Therefore, the limit is \(\frac{1}{2}\).

Answer

\(\frac{1}{2}\)
51011312
Evaluate \(\lim_{x\to\infty}\frac{(2x-1)(x+4)}{5x^2-3x}\).

Hints

- Expand the numerator first. - Divide by the highest power of \(x\). - Identify which terms approach zero.

Solution

1. Expand the numerator: \((2x-1)(x+4)=2x^2+7x-4\). 2. Divide the numerator and denominator by \(x^2\): \(\frac{2+\frac{7}{x}-\frac{4}{x^2}}{5-\frac{3}{x}}\). 3. As \(x\to\infty\), the terms containing \(\frac{1}{x}\) or \(\frac{1}{x^2}\) approach \(0\). 4. Therefore, the limit is \(\frac{2}{5}\).

Answer

\(\frac{2}{5}\)
52180012
Let \(g(x)=\frac{10}{x^3+5x}\). Find the horizontal asymptote as \(x\to\pm\infty\). Justify your answer by comparing the growth of the numerator and denominator.

Hints

- Compare the degrees of the numerator and denominator. - Identify the dominant term in the denominator. - Decide what happens when a fixed number is divided by an increasingly large magnitude.

Solution

1. The numerator is constant, while the denominator is a third-degree polynomial. 2. As \(|x|\to\infty\), the term \(x^3\) dominates the denominator, so \(|x^3+5x|\to\infty\). 3. A constant divided by a quantity whose magnitude grows without bound approaches \(0\). Thus, \(g(x)\to0\) as \(x\to\infty\) and as \(x\to-\infty\). 4. Therefore, the horizontal asymptote is the x-axis.

Answer

\(y=0\)
52180112
Analyze each function as \(x\to\infty\). Match it to one description. (1) Approaches a finite limit (2) Increases without bound toward \(+\infty\) (3) Decreases without bound toward \(-\infty\) (4) Oscillates and has no limit A: \(f(x)=10-x^2\) B: \(g(x)=2\sin(x)+1\) C: \(h(x)=\frac{5x+2}{x}\) D: \(k(x)=e^{0.5x}\)

Hints

- Identify the term that controls each function for large positive \(x\). - Recall the range and repeated behavior of sine. - Simplify the rational expression before taking its limit.

Solution

1. For A, the term \(-x^2\) dominates, so \(f(x)\to-\infty\). Thus, A matches (3). 2. For B, the outputs continue to oscillate between \(-1\) and \(3\), so no limit exists. Thus, B matches (4). 3. Rewrite C as \(h(x)=5+\frac{2}{x}\). Since \(\frac{2}{x}\to0\), \(h(x)\to5\). Thus, C matches (1). 4. For D, the exponent grows without bound, so \(e^{0.5x}\to+\infty\). Thus, D matches (2).

Answer

A: (3) B: (4) C: (1) D: (2)
52572712
Consider the sequence \((a_n)\) defined by \(a_n=\frac{n+1}{n}(-1)^n\) for positive integers \(n\). 1) Find the first six terms \(a_1\) through \(a_6\). 2) Find all accumulation points of the sequence. 3) Explain why the sequence does not converge.

Hints

- Track how the sign changes with the index. - Analyze the fractional factor and the sign factor separately. - Compare the behavior of the even-indexed and odd-indexed subsequences. - Recall the relationship between accumulation points and a sequence limit.

Solution

1. Substitution gives \(a_1=-2\), \(a_2=1.5\), \(a_3=-\frac43\approx-1.33\), \(a_4=1.25\), \(a_5=-1.2\), and \(a_6=\frac76\approx1.17\). 2. For even indices \(n=2k\), \(a_{2k}=\frac{2k+1}{2k}=1+\frac{1}{2k}\to1\). For odd indices \(n=2k-1\), \(a_{2k-1}=-\frac{2k}{2k-1}=-\left(1+\frac{1}{2k-1}\right)\to-1\). Therefore, the accumulation points are \(1\) and \(-1\). 3. A convergent sequence has only one accumulation point, its limit. Since this sequence has two distinct accumulation points, it diverges.

Answer

1) \(a_1=-2\), \(a_2=1.5\), \(a_3=-\frac43\), \(a_4=1.25\), \(a_5=-1.2\), and \(a_6=\frac76\). 2) The accumulation points are \(1\) and \(-1\). 3) The sequence diverges because it has two distinct accumulation points.
52573512
Consider the sequence \((a_n)\) defined by \(a_n=\frac{2n+1}{n}\) for positive integers \(n\). 1) Write the first six terms \(a_1\) through \(a_6\) as decimals. 2) Find the limit \(g\) as \(n\to\infty\). 3) Find the first index from which \(|a_n-g|<0.05\). 4) Prove that the sequence is strictly decreasing and that every term is greater than \(2\).

Hints

- Rewrite the sequence term so that the limit is easy to identify. - Express closeness to the limit with an absolute-value inequality. - Compare consecutive terms to establish monotonicity. - Use the sign of the remaining fraction to compare each term with \(2\).

Solution

1. The first six terms are \(3\), \(2.5\), \(2.33\), \(2.25\), \(2.2\), and \(2.17\), with the repeating decimals rounded to the nearest hundredth. 2. Rewrite \(a_n=2+\frac1n\). Since \(\frac1n\to0\), the limit is \(g=2\). 3. Solve \(|a_n-2|=\frac1n<0.05\). This requires \(n>20\), so the condition holds beginning at \(n=21\). 4. The difference of consecutive terms is \(a_{n+1}-a_n=\frac{1}{n+1}-\frac1n=-\frac{1}{n(n+1)}<0\). Thus, the sequence is strictly decreasing. Also, \(a_n=2+\frac1n>2\) for every positive integer \(n\).

Answer

1) \(3\), \(2.5\), \(2.33\), \(2.25\), \(2.2\), and \(2.17\). 2) \(g=2\). 3) The condition holds beginning at \(n=21\). 4) The sequence is strictly decreasing, and \(a_n>2\) for every positive integer \(n\).
52179312
Let \(f(x)=2+\frac{4}{x-5}\). The horizontal asymptote of its graph is the center line of a horizontal band with total width \(0.2\). Determine whether the graph is inside or outside the band at each input. 1. \(x=15\) 2. \(x=55\)

Hints

- Find the horizontal asymptote. - Use half of the total band width as the maximum allowed vertical distance. - Compare \(|f(x)-2|\) with that distance.

Solution

1. The horizontal asymptote is \(y=2\). A band of total width \(0.2\) extends \(0.1\) above and below its center line, so a point is inside when \(|f(x)-2|<0.1\). 2. At \(x=15\), \(|f(15)-2|=\left|\frac{4}{10}\right|=0.4\). Since \(0.4>0.1\), the graph is outside the band. 3. At \(x=55\), \(|f(55)-2|=\left|\frac{4}{50}\right|=0.08\). Since \(0.08<0.1\), the graph is inside the band.

Answer

1. Outside 2. Inside
52179412
Let \(g(x)=\frac{10}{2x+1}+5\). The horizontal asymptote of its graph is the center line of a horizontal band with total width \(0.5\). Verify algebraically that: 1. The point on the graph at \(x=10\) is outside the band. 2. Every point on the graph with \(x>19.5\) is inside the band.

Hints

- Find the horizontal asymptote and half the band width. - Evaluate the vertical distance from the graph to the asymptote at \(x=10\). - Solve the distance inequality for \(x\).

Solution

1. The horizontal asymptote is \(y=5\). The band extends \(0.25\) above and below it, so points inside satisfy \(|g(x)-5|<0.25\). 2. At \(x=10\), \(|g(10)-5|=\frac{10}{21}\approx0.476\). Since this is greater than \(0.25\), the point is outside. 3. For \(x>19.5\), the denominator \(2x+1\) is positive. Solve \(\frac{10}{2x+1}<0.25\): \(10<0.25(2x+1)\), so \(40<2x+1\), which gives \(x>19.5\). 4. Thus, every graph point with \(x>19.5\) is inside the band.

Answer

1. Outside, because \(\frac{10}{21}\approx0.476>0.25\). 2. The inequality \(|g(x)-5|<0.25\) holds for every \(x>19.5\).
52179512
Let \(f(x)=1.5-\frac{2}{x-1}\), with its maximal real domain. a) State the equations of the vertical and horizontal asymptotes. b) A horizontal band of total width \(0.8\) is centered on the horizontal asymptote. Determine whether the graph points at \(x=3\) and \(x=7\) are inside or outside the band.

Hints

- Find where the denominator is zero and what value the function approaches as \(|x|\) grows. - Use half the total band width. - Compare each vertical distance from the horizontal asymptote with \(0.4\).

Solution

1. The denominator is zero at \(x=1\), so the vertical asymptote is \(x=1\). As \(|x|\to\infty\), the fraction approaches \(0\), so the horizontal asymptote is \(y=1.5\). 2. The band extends \(0.4\) above and below \(y=1.5\). A point is inside when \(|f(x)-1.5|<0.4\). 3. At \(x=3\), \(f(3)=1.5-\frac{2}{2}=0.5\). Its distance from the asymptote is \(1\), so it is outside. 4. At \(x=7\), \(f(7)=1.5-\frac{2}{6}=\frac{7}{6}\approx1.17\). Its distance from the asymptote is \(\frac{1}{3}<0.4\), so it is inside.

Answer

a) Vertical asymptote: \(x=1\); horizontal asymptote: \(y=1.5\) b) At \(x=3\): outside. At \(x=7\): inside.
52179612
Let \(g(x)=\frac{3}{0.5x+1}-2\), where \(x\ne-2\). a) Find the vertical and horizontal asymptotes. b) For \(x>-2\), determine when \(g(x)\) differs from the horizontal asymptote by less than \(0.1\). Solve an inequality to give the interval of x-values.

Hints

- Find where the denominator is zero and what output the function approaches at infinity. - Express vertical distance from the horizontal asymptote with an absolute value. - Use the condition \(x>-2\) to determine the sign of the denominator.

Solution

1. The denominator is zero at \(0.5x+1=0\), so the vertical asymptote is \(x=-2\). As \(|x|\to\infty\), the fraction approaches \(0\), so the horizontal asymptote is \(y=-2\). 2. The required condition is \(|g(x)+2|<0.1\), or \(\left|\frac{3}{0.5x+1}\right|<0.1\). 3. For \(x>-2\), the denominator is positive, so \(\frac{3}{0.5x+1}<0.1\). 4. Multiply by the positive denominator: \(3<0.1(0.5x+1)=0.05x+0.1\). Thus, \(2.9<0.05x\), which gives \(x>58\).

Answer

a) Vertical asymptote: \(x=-2\); horizontal asymptote: \(y=-2\) b) \(x>58\)
52179712
Let \(f(x)=10(3^{-x})+4\). Find the horizontal asymptote as \(x\to\infty\). A horizontal band of total width \(0.5\) is centered on this asymptote. Determine whether the graph is inside or outside the band at \(x=3\) and \(x=4\).

Hints

- Determine what happens to \(3^{-x}\) as \(x\) grows. - Use half the total band width. - Compare each output's distance from \(4\) with \(0.25\).

Solution

1. Since \(3^{-x}\to0\) as \(x\to\infty\), \(f(x)\to4\). The horizontal asymptote is \(y=4\). 2. The band extends \(0.25\) above and below the asymptote, so points inside satisfy \(|f(x)-4|<0.25\). 3. At \(x=3\), \(f(3)=4+\frac{10}{27}\approx4.37\). Its distance from the asymptote is about \(0.37>0.25\), so it is outside. 4. At \(x=4\), \(f(4)=4+\frac{10}{81}\approx4.12\). Its distance from the asymptote is about \(0.12<0.25\), so it is inside.

Answer

Horizontal asymptote: \(y=4\) At \(x=3\): outside At \(x=4\): inside
52179812
Let \(g(x)=5-2^{2-x}\). Find \(\lim_{x\to\infty}g(x)\). A horizontal band of total width \(0.1\) is centered on the horizontal asymptote. Show that the graph point at \(x=6\) is outside the band while the graph point at \(x=8\) is inside.

Hints

- Rewrite \(2^{2-x}\) as a constant times an exponential decay factor. - Use half the total band width. - Compare the exact distances from the asymptote.

Solution

1. Since \(2^{2-x}=4\left(\frac{1}{2}\right)^x\to0\), the limit is \(5\), and the horizontal asymptote is \(y=5\). 2. The band extends \(0.05\) above and below the asymptote, so points inside satisfy \(|g(x)-5|<0.05\). 3. \(g(6)=5-2^{-4}=5-0.0625=4.9375\). The distance from \(5\) is \(0.0625>0.05\), so the point is outside. 4. \(g(8)=5-2^{-6}=5-0.015625=4.984375\). The distance from \(5\) is \(0.015625<0.05\), so the point is inside.

Answer

The limit is \(5\). At \(x=6\): outside At \(x=8\): inside
52179912
Let \(f(x)=\frac{4x+1}{2x^2-3}\). Analyze the behavior of \(f(x)\) as \(x\to\infty\) and as \(x\to-\infty\). Justify your answer by rewriting the expression so the limits of its terms are clear.

Hints

- Divide by the highest power of \(x\) in the denominator. - Evaluate the limits of the resulting reciprocal-power terms. - Treat positive and negative infinity separately, then compare the results.

Solution

1. Divide the numerator and denominator by \(x^2\): \(f(x)=\frac{\frac{4}{x}+\frac{1}{x^2}}{2-\frac{3}{x^2}}\). 2. As \(x\to\infty\) or \(x\to-\infty\), each term containing \(\frac{1}{x}\) or \(\frac{1}{x^2}\) approaches \(0\). 3. Therefore, the numerator approaches \(0\), the denominator approaches \(2\), and \(f(x)\to0\) in both directions.

Answer

\(\lim_{x\to\infty}f(x)=0\) and \(\lim_{x\to-\infty}f(x)=0\)
52180212
Give one example of a function with each behavior as \(x\to\infty\). Use a different function family for each part. a) The function oscillates forever between \(2\) and \(4\), so it has no limit. b) The function approaches \(+\infty\) and grows faster than every linear function. c) The function approaches \(-\infty\).

Hints

- Shift a sine or cosine function to obtain the required oscillation interval. - A polynomial of degree greater than \(1\) can outgrow every linear function. - Reflect a positive growth function across the x-axis to obtain behavior toward \(-\infty\).

Solution

1. For part a, \(f(x)=\sin(x)+3\) is trigonometric and has range \([2, 4]\), so it continues to oscillate without a limit. 2. For part b, \(g(x)=x^2\) is polynomial and approaches \(+\infty\) faster than any linear function. 3. For part c, \(h(x)=-e^x\) is exponential and approaches \(-\infty\).

Answer

Possible answers: a) \(f(x)=\sin(x)+3\) b) \(g(x)=x^2\) c) \(h(x)=-e^x\)
52180312
Let \(f(x)=\frac{5}{x-3}+1\). a) Find the horizontal asymptote. b) A horizontal band of total width \(0.2\) is centered on the asymptote. Find a threshold \(x_0\) such that the graph is inside the band for every \(x>x_0\). c) Find a threshold \(x_1\) such that the graph is inside the band for every \(x<x_1\).

Hints

- Use half the total band width. - Express the distance from the horizontal asymptote with an absolute value. - Solve the resulting absolute-value inequality.

Solution

1. Since \(\frac{5}{x-3}\to0\) as \(x\to\pm\infty\), the horizontal asymptote is \(y=1\). 2. The band extends \(0.1\) above and below the asymptote, so the condition is \(\left|\frac{5}{x-3}\right|<0.1\). 3. This is equivalent to \(|x-3|>50\), giving \(x>53\) or \(x<-47\). 4. Therefore, one suitable pair of thresholds is \(x_0=53\) and \(x_1=-47\).

Answer

a) \(y=1\) b) \(x_0=53\) c) \(x_1=-47\)
52180412
Consider the family \(g_a(x)=\frac{a}{x}+2\), where \(a>0\). The horizontal asymptote is \(y=2\). A horizontal band of total width \(0.01\) is centered on the asymptote. For positive \(x\), determine how \(a\) affects the threshold \(x_{\text{start}}\) such that every point with \(x>x_{\text{start}}\) lies inside the band. Explain the relationship.

Hints

- Use half the total band width as the allowed deviation. - Write an inequality involving \(\frac{a}{x}\). - Solve for \(x\) while using \(a>0\) and \(x>0\).

Solution

1. The band extends \(0.005\) above and below \(y=2\). For \(x>0\), the condition is \(g_a(x)-2<0.005\). 2. Substitute the formula: \(\frac{a}{x}<0.005\). 3. Since \(a>0\) and \(x>0\), this is equivalent to \(x>\frac{a}{0.005}=200a\). 4. Thus, \(x_{\text{start}}=200a\). The threshold is directly proportional to \(a\): doubling \(a\) doubles the required threshold.

Answer

\(x_{\text{start}}=200a\); the threshold is directly proportional to \(a\).
52180512
Give one rational function \(f\) and one exponential function \(g\) such that each approaches \(-2\) as \(x\to\infty\). For each example, analyze its behavior as \(x\to-\infty\) and find its real zeros, if any.

Hints

- For a rational function with equal numerator and denominator degrees, use the ratio of leading coefficients. - Shift an exponential decay function downward. - Set each function equal to zero to find its zeros.

Solution

1. One rational example is \(f(x)=\frac{-2x}{x+1}\). The ratio of leading coefficients gives \(f(x)\to-2\) as \(x\to\infty\) and as \(x\to-\infty\). Its zero is \(x=0\). 2. One exponential example is \(g(x)=e^{-x}-2\). As \(x\to\infty\), \(e^{-x}\to0\), so \(g(x)\to-2\). As \(x\to-\infty\), \(e^{-x}\to+\infty\), so \(g(x)\to+\infty\). 3. Solve \(e^{-x}-2=0\): \(e^{-x}=2\), so \(-x=\ln(2)\) and \(x=-\ln(2)\).

Answer

Possible examples: \(f(x)=\frac{-2x}{x+1}\): as \(x\to-\infty\), \(f(x)\to-2\); zero at \(x=0\). \(g(x)=e^{-x}-2\): as \(x\to-\infty\), \(g(x)\to+\infty\); zero at \(x=-\ln(2)\).
52180712
Decide whether the statement is true or false, and justify your answer. “If the graph of \(f\) stays above the line \(y=2\) for every \(x>0\), and the function values decrease as \(x\) increases, then \(f(x)\to2\) as \(x\to\infty\).”

Hints

- Distinguish between a lower bound and the actual limit. - Try a decreasing reciprocal function shifted upward. - One counterexample is enough to disprove a universal statement.

Solution

1. The statement is false. Being decreasing and bounded below by \(2\) does not force the limit to equal that particular lower bound. 2. Consider \(f(x)=3+\frac{1}{x}\) for \(x>0\). This function is decreasing and satisfies \(f(x)>3>2\). 3. However, \(\lim_{x\to\infty}\left(3+\frac{1}{x}\right)=3\), not \(2\). This counterexample disproves the statement.

Answer

False. A counterexample is \(f(x)=3+\frac{1}{x}\), which decreases and stays above \(2\) but approaches \(3\).
52180812
Let \(f(x)=\frac{4x-1}{x+1}\) for \(x>0\). 1. Find \(L=\lim_{x\to\infty}f(x)\). 2. Show algebraically that \(f(x)<5\) for every \(x>0\). 3. Explain why these results disprove the statement: “Because \(f\) is increasing and bounded above by \(5\), its limit must be \(5\).”

Hints

- Divide by \(x\) to find the limit. - Use the positive denominator to clear the inequality safely. - Distinguish an arbitrary upper bound from the value approached by the function.

Solution

1. Divide the numerator and denominator by \(x\): \(f(x)=\frac{4-\frac{1}{x}}{1+\frac{1}{x}}\). Therefore, \(L=4\). 2. Since \(x+1>0\), \(\frac{4x-1}{x+1}<5\) is equivalent to \(4x-1<5x+5\), or \(x>-6\). This is true for every \(x>0\). 3. The function can also be written as \(f(x)=4-\frac{5}{x+1}\), which increases toward \(4\). Although \(5\) is an upper bound, it is not the limit. An upper bound need not be the least upper bound or the value approached by the function.

Answer

1. \(L=4\) 2. \(f(x)<5\) for every \(x>0\). 3. The statement is false because the actual limit is \(4\), even though \(5\) is an upper bound.
52571312
Consider the sequence \((a_n)\) defined by \(a_n=\frac{4n-1}{2n+3}\) for positive integers \(n\). 1) Find \(a_1\), \(a_2\), and \(a_{10}\). 2) Find the limit \(g\) as \(n\to\infty\). 3) Find the least positive integer \(n_0\) such that \(|a_n-g|<0.001\) for every \(n\ge n_0\). 4) Show that the sequence is strictly increasing.

Hints

- Divide the numerator and denominator by the highest power of \(n\). - Express the distance from a sequence term to the limit with an absolute value. - Solve the resulting inequality for \(n\). - To study monotonicity, compare consecutive terms.

Solution

1. Substitute the indices: \(a_1=\frac35=0.6\), \(a_2=\frac77=1\), and \(a_{10}=\frac{39}{23}\approx1.696\). 2. Divide the numerator and denominator by \(n\): \(a_n=\frac{4-\frac1n}{2+\frac3n}\to\frac42=2\). Thus, \(g=2\). 3. Solve the error inequality: \(\left|\frac{4n-1}{2n+3}-2\right|=\frac{7}{2n+3}<0.001\). This gives \(7000<2n+3\), so \(n>3498.5\). Therefore, the least possible value is \(n_0=3499\). 4. Compute the difference of consecutive terms: \(a_{n+1}-a_n=\frac{4n+3}{2n+5}-\frac{4n-1}{2n+3}=\frac{14}{(2n+5)(2n+3)}>0\). Therefore, the sequence is strictly increasing.

Answer

1) \(a_1=0.6\), \(a_2=1\), and \(a_{10}\approx1.696\). 2) \(g=2\). 3) \(n_0=3499\). 4) Since \(a_{n+1}-a_n>0\), the sequence is strictly increasing.
52571412
Consider the sequence \((b_n)\) defined by \(b_n=\frac{n^2-10}{n+5}\) for positive integers \(n\). 1) Describe the behavior of the sequence as \(n\to\infty\) using limit notation. 2) Explain why the sequence is strictly increasing from some index onward. 3) Find the least positive integer \(n\) for which \(b_n>50\).

Hints

- Compare the degrees of the numerator and denominator. - You may analyze the derivative of the associated real-valued function or compare consecutive terms. - Rewrite the condition as a quadratic inequality. - Find the zeros of the quadratic expression.

Solution

1. Polynomial division gives \(b_n=n-5+\frac{15}{n+5}\). The linear term grows without bound while the fraction approaches \(0\), so \(\lim_{n\to\infty}b_n=\infty\). 2. Compare consecutive terms: \(b_{n+1}-b_n=\frac{(n+1)^2-10}{n+6}-\frac{n^2-10}{n+5}=\frac{n^2+11n+15}{(n+5)(n+6)}\). For every positive integer \(n\), both the numerator and denominator are positive. Therefore, the sequence is strictly increasing for all positive integers \(n\). 3. Solve \(\frac{n^2-10}{n+5}>50\). Since \(n+5>0\), this is equivalent to \(n^2-50n-260>0\). The positive root is \(25+\sqrt{885}\approx54.75\), so the least positive integer solution is \(n=55\).

Answer

1) \(\lim_{n\to\infty}b_n=\infty\). 2) Since \(b_{n+1}-b_n>0\) for every positive integer \(n\), the sequence is strictly increasing. 3) The least value is \(n=55\).
52572112
Consider the sequence \(a_n=4+\frac{10(-1)^n}{n}\) for positive integers \(n\). 1) Find \(a_1\), \(a_2\), \(a_{10}\), and \(a_{100}\). 2) Find the limit \(g\) as \(n\to\infty\). 3) Find the first index from which every remaining term lies in the interval \((3.95, 4.05)\). 4) Explain why the sequence is bounded but not monotonic.

Hints

- Determine what happens to the fractional term as \(n\) grows. - Rewrite the interval condition as \(|a_n-g|<\varepsilon\). - Review the definition of a monotonic sequence. - To show boundedness, find fixed lower and upper bounds for all terms.

Solution

1. Substitution gives \(a_1=-6\), \(a_2=9\), \(a_{10}=5\), and \(a_{100}=4.1\). 2. Since \(\frac{10(-1)^n}{n}\to0\), the sequence converges to \(g=4\). 3. The interval condition is equivalent to \(|a_n-4|<0.05\). Thus, \(\left|\frac{10(-1)^n}{n}\right|=\frac{10}{n}<0.05\). This requires \(n>200\), so every term beginning with \(a_{201}\) lies in the interval. 4. The odd-indexed terms lie below \(4\), and the even-indexed terms lie above \(4\), so the sequence is not monotonic. Also, the largest deviation occurs among the first terms: \(-6\le a_n\le9\). Therefore, it is bounded.

Answer

1) \(a_1=-6\), \(a_2=9\), \(a_{10}=5\), and \(a_{100}=4.1\). 2) \(g=4\). 3) Every remaining term lies in the interval beginning at \(n=201\). 4) The sequence is bounded by \(-6\) and \(9\), but it alternates around its limit and is not monotonic.
52573012
Consider the sequence \((b_n)\) defined by \(b_n=\frac{n^2-1}{2n^2}\) for positive integers \(n\). 1) Rewrite the expression to show that the sequence converges to \(0.5\). 2) Find all positive integers \(n\) for which \(|b_n-0.5|<0.01\). 3) Explain why \(0\) is a lower bound and \(0.5\) is an upper bound for the sequence.

Hints

- Split the fraction so that \(n\) appears in only one term. - Use the rewritten form to compute the distance from the limit. - Check the smallest allowed index and how the fractional term changes as \(n\) increases.

Solution

1. Rewrite the term: \(b_n=\frac{n^2}{2n^2}-\frac{1}{2n^2}=\frac12-\frac{1}{2n^2}=0.5-\frac{1}{2n^2}\). Since \(\frac{1}{2n^2}\to0\), \(b_n\to0.5\). 2. Solve \(|b_n-0.5|=\frac{1}{2n^2}<0.01\). This gives \(n^2>50\), so \(n>\sqrt{50}\approx7.071\). Therefore, the condition holds for every integer \(n\ge8\). 3. For every \(n\ge1\), \(\frac{1}{2n^2}>0\), so \(b_n<0.5\). Also, \(b_1=0\), and \(\frac{1}{2n^2}\le\frac12\), so \(b_n\ge0\). Thus, \(0\le b_n<0.5\).

Answer

1) \(b_n=0.5-\frac{1}{2n^2}\), so the limit is \(0.5\). 2) The condition holds for every \(n\ge8\). 3) The sequence satisfies \(0\le b_n<0.5\).
52573612
Consider the sequence \((b_n)\) defined by \(b_n=\frac{3n-1}{n+1}\) for positive integers \(n\). 1) Show that the sequence is strictly increasing. 2) Give an upper bound and justify it. 3) Find the limit \(L\). 4) For any \(\varepsilon>0\), give a formula for the least positive integer \(n_0\) such that \(|L-b_n|<\varepsilon\) for every \(n\ge n_0\).

Hints

- Compare \(b_n\) and \(b_{n+1}\). - An upper bound is a number no sequence term exceeds. - Divide by the highest power of \(n\) to find the limit. - Solve the final inequality for \(n\), paying attention to the strict inequality.

Solution

1. Compare consecutive terms: \(b_{n+1}-b_n=\frac{3n+2}{n+2}-\frac{3n-1}{n+1}=\frac{4}{(n+1)(n+2)}>0\). Therefore, the sequence is strictly increasing. 2. Since \(b_n=\frac{3n-1}{n+1}<\frac{3n+3}{n+1}=3\), the number \(3\) is an upper bound. 3. Dividing the numerator and denominator by \(n\) gives \(L=\lim_{n\to\infty}\frac{3-\frac1n}{1+\frac1n}=3\). 4. The error is \(|3-b_n|=\frac{4}{n+1}\). The inequality \(\frac{4}{n+1}<\varepsilon\) is equivalent to \(n>\frac{4}{\varepsilon}-1\). Therefore, the least positive integer that works is \(n_0=\max\left(1, \left\lfloor\frac{4}{\varepsilon}-1\right\rfloor+1\right)\).

Answer

1) \(b_{n+1}-b_n=\frac{4}{(n+1)(n+2)}>0\), so the sequence is strictly increasing. 2) An upper bound is \(3\). 3) \(L=3\). 4) \(n_0=\max\left(1, \left\lfloor\frac{4}{\varepsilon}-1\right\rfloor+1\right)\).
52573712
Consider the sequence \((a_n)\) defined by \(a_n=\sqrt{n^2+4n}\) for positive integers \(n\). 1) Explain why the sequence is strictly increasing. 2) Show that the sequence is unbounded above by giving, for an arbitrary real number \(M>0\), a condition on \(n\) that guarantees \(a_n>M\). 3) Find the least positive integer \(n\) such that every term from that index onward is greater than \(100\).

Hints

- Compare the expressions under the square roots for consecutive indices. - Square the inequality carefully when both sides are positive. - Solve the resulting quadratic inequality for \(n\). - A simpler lower comparison may also establish unbounded growth.

Solution

1. Since \(a_{n+1}^2-a_n^2=[(n+1)^2+4(n+1)]-(n^2+4n)=2n+5>0\), and both terms are nonnegative, \(a_{n+1}>a_n\). Thus, the sequence is strictly increasing. 2. The inequality \(a_n>M\) is equivalent to \(n^2+4n>M^2\), or \((n+2)^2>M^2+4\). Since \(n+2>0\), it is enough to choose \(n>-2+\sqrt{M^2+4}\). Such an integer exists for every \(M>0\), so the sequence is unbounded above. 3. For \(M=100\), the condition becomes \(n>-2+\sqrt{10{,}004}\approx98.02\). Therefore, the least positive integer is \(n=99\). Since the sequence is increasing, every later term is also greater than \(100\).

Answer

1) The sequence is strictly increasing. 2) It is enough to choose \(n>-2+\sqrt{M^2+4}\). 3) The least index is \(n=99\).
52574712
Consider the sequence \((a_n)\) defined by \(a_n=15-\frac14n^2\) for positive integers \(n\). 1) Show that the sequence is strictly decreasing. 2) Show that the sequence is unbounded below and diverges to \(-\infty\). 3) Find the least index from which every term is less than \(-200\). 4) Find the least index from which every term is less than \(-5000\).

Hints

- Compare consecutive terms. - Translate unbounded decrease into a condition involving an arbitrary positive number \(M\). - Set up an inequality for each requested threshold. - Remember that the index must be a positive integer.

Solution

1. Compare consecutive terms: \(a_{n+1}-a_n=-\frac14[(n+1)^2-n^2]=-\frac12n-\frac14<0\). Therefore, the sequence is strictly decreasing. 2. Let \(M>0\). The condition \(a_n<-M\) is equivalent to \(15-\frac14n^2<-M\), so \(n^2>4(15+M)\). Thus, any integer satisfying \(n>2\sqrt{15+M}\) makes \(a_n<-M\). Therefore, the sequence is unbounded below and \(a_n\to-\infty\). 3. Solve \(15-\frac14n^2<-200\). This gives \(n^2>860\), so \(n>\sqrt{860}\approx29.33\). The least index is \(n=30\). 4. Solve \(15-\frac14n^2<-5000\). This gives \(n^2>20{,}060\), so \(n>\sqrt{20{,}060}\approx141.63\). The least index is \(n=142\).

Answer

1) The sequence is strictly decreasing. 2) For every \(M>0\), \(a_n<-M\) whenever \(n>2\sqrt{15+M}\), so \(a_n\to-\infty\). 3) Beginning at \(n=30\). 4) Beginning at \(n=142\).
52575612
A regular \(n\)-gon is inscribed in a circle of radius \(R\), and its perimeter is \(p_n\). a) Find a formula for \(p_n\) in terms of \(n\) and \(R\). b) Let \(v_n=\frac{p_{2n}}{p_n}\). Show that \(v_n=\frac{1}{\cos\left(\frac{\pi}{2n}\right)}\). c) Find \(\lim_{n\to\infty}v_n\) and interpret the result in terms of the circumference approximation.

Hints

- Use trigonometry in one isosceles triangular section to find the side length. - Apply the double-angle identity for sine in part b). - Determine what happens to the cosine argument as \(n\) grows.

Solution

1. The central angle subtended by one side is \(\frac{2\pi}{n}\). Bisecting the corresponding isosceles triangle gives side length \(s_n=2R\sin\left(\frac{\pi}{n}\right)\). Therefore, \(p_n=2nR\sin\left(\frac{\pi}{n}\right)\). 2. Replacing \(n\) by \(2n\), \(p_{2n}=4nR\sin\left(\frac{\pi}{2n}\right)\). Thus, \(v_n=\frac{2\sin(\pi/(2n))}{\sin(\pi/n)}\). 3. Using \(\sin(2x)=2\sin x\cos x\) with \(x=\frac{\pi}{2n}\), \(\sin\left(\frac{\pi}{n}\right)=2\sin\left(\frac{\pi}{2n}\right)\cos\left(\frac{\pi}{2n}\right)\). Therefore, \(v_n=\frac{1}{\cos\left(\frac{\pi}{2n}\right)}\). 4. Since \(\frac{\pi}{2n}\to0\), \(\lim_{n\to\infty}v_n=\frac{1}{\cos0}=1\). The relative increase \(\frac{p_{2n}-p_n}{p_n}=v_n-1\) approaches \(0\), so doubling the number of sides produces a vanishing relative improvement as the polygonal perimeter approaches the circle's circumference.

Answer

a) \(p_n=2nR\sin\left(\frac{\pi}{n}\right)\). b) \(v_n=\frac{1}{\cos\left(\frac{\pi}{2n}\right)}\). c) The limit is \(1\); the relative perimeter increase from doubling the number of sides approaches \(0\).
53409712
Lukas views the graph of a function \(f\) only on the interval \(0\le x\le15\). He says, “The graph clearly shows that the function values approach \(0\) as \(x\) increases, so \(\lim_{x\to\infty}f(x)=0\).” Is Lukas’s conclusion justified from the displayed graph alone? Explain.
Figure for problem 534097

Hints

- Consider what the graph might do after it leaves the right edge of the window. - A decreasing curve does not necessarily continue decreasing forever. - Distinguish visual evidence from a mathematical proof of a limit.

Solution

1. In the visible interval, the graph decreases and moves closer to the x-axis. 2. A finite graphing window does not determine what happens for all larger x-values. The graph could later turn upward, continue downward, or approach a different horizontal asymptote. 3. For example, \(p(x)=\frac{10}{x+1}+0.02x\) decreases throughout the displayed interval but eventually rises without bound because the linear term dominates. Thus, a graph that looks as though it approaches \(0\) in one window need not have limit \(0\). 4. Therefore, Lukas’s conclusion is not justified without additional information, such as a function rule or a proof.

Answer

No. A finite graphing window can suggest a limit but cannot establish the behavior as \(x\to\infty\). The graph may behave differently outside the displayed interval.
53444812
The graph shows the function \(f(x) = \sqrt{x+1}-1\). Secant line \(s\) connects the origin \(O(0, 0)\) to point \(P(3, 1)\) on the graph. a) Calculate the slope of secant line \(s\). b) The dashed line \(t\) is tangent to the graph at the origin. Find its slope from the graph. c) Use the graph and your results from parts a) and b) to determine \(\lim_{x\to 0}\frac{\sqrt{x+1}-1}{x}\). Explain your reasoning.
Figure for problem 534448

Hints

- Use two convenient points on the dashed line to find its slope. - Think about how the secant line changes as point \(P\) moves along the curve toward \(O\). - The expression in the limit is a difference quotient at \(x=0\).

Solution

1. The secant slope is \(m_s = \frac{f(3)-f(0)}{3-0} = \frac{1-0}{3} = \frac{1}{3}\). 2. The tangent line passes through \((0, 0)\) and \((2, 1)\), so its slope is \(m_t = \frac{1-0}{2-0} = 0.5\). 3. The expression \(\frac{\sqrt{x+1}-1}{x}\) is the slope of the secant line through \(O\) and \((x,f(x))\). As \(x\to 0\), the secant line approaches the tangent line at the origin. 4. Therefore, the secant slopes approach the tangent slope, so \(\lim_{x\to 0}\frac{\sqrt{x+1}-1}{x} = 0.5\).

Answer

a) \(m_s = \frac{1}{3}\) b) \(m_t = 0.5\) c) The limit is the tangent slope at the origin, so \(\lim_{x\to 0}\frac{\sqrt{x+1}-1}{x} = 0.5\).
52572212
Consider the sequence \(a_n=\frac{3n^2}{n^2+2}\) for positive integers \(n\). 1) Show algebraically that the sequence is strictly increasing. 2) Find the limit \(g\). 3) Find the least integer \(N\) such that \(|a_n-g|<0.001\) for every \(n>N\). 4) Give a lower bound and an upper bound for the set of sequence terms.

Hints

- Compare \(a_{n+1}\) and \(a_n\) by subtracting. - Divide the numerator and denominator by the highest power of \(n\). - Solve \(|a_n-g|<\varepsilon\) for \(n\). - Use monotonicity and the limit to identify bounds.

Solution

1. Compare consecutive terms: \(a_{n+1}-a_n=\frac{3(n+1)^2}{(n+1)^2+2}-\frac{3n^2}{n^2+2}\) \(=\frac{12n+6}{((n+1)^2+2)(n^2+2)}>0\). Therefore, the sequence is strictly increasing. 2. Divide the numerator and denominator by \(n^2\): \(a_n=\frac{3}{1+\frac{2}{n^2}}\to3\). Thus, \(g=3\). 3. Solve \(\left|\frac{3n^2}{n^2+2}-3\right|=\frac{6}{n^2+2}<0.001\). This requires \(n^2>5998\), or \(n>\sqrt{5998}\approx77.45\). Since the condition must hold for every \(n>N\), the least value is \(N=77\). 4. The first term is \(a_1=1\). Because the sequence is strictly increasing and converges to \(3\), \(1\) is a lower bound and \(3\) is an upper bound.

Answer

1) \(a_{n+1}-a_n>0\), so the sequence is strictly increasing. 2) \(g=3\). 3) \(N=77\), so the condition holds for every \(n\ge78\). 4) A lower bound is \(1\), and an upper bound is \(3\).
52574812
Consider the sequence \((b_n)\) defined by \(b_n=\frac{3n^2-1000}{n}\) for positive integers \(n\). 1) Describe the behavior of the sequence as \(n\to\infty\). 2) Find the least positive integer \(n_0\) such that \(b_n>100\) for every \(n\ge n_0\). 3) Find the least positive integer \(n_1\) such that \(b_n>1000\) for every \(n\ge n_1\).

Hints

- Divide each term in the numerator by the denominator. - Identify which part controls the behavior for large \(n\). - Convert each rational inequality into a quadratic inequality. - Check the least integer above the positive zero.

Solution

1. Rewrite \(b_n=3n-\frac{1000}{n}\). As \(n\to\infty\), the first term tends to \(\infty\) and the second tends to \(0\). Therefore, \(\lim_{n\to\infty}b_n=\infty\). 2. Solve \(3n-\frac{1000}{n}>100\). Since \(n>0\), this is equivalent to \(3n^2-100n-1000>0\). The positive zero is \(\frac{100+\sqrt{22{,}000}}{6}\approx41.39\). Thus, the least possible value is \(n_0=42\). 3. Similarly, \(b_n>1000\) is equivalent to \(3n^2-1000n-1000>0\). The positive zero is \(\frac{1000+\sqrt{1{,}012{,}000}}{6}\approx334.33\). Therefore, \(n_1=335\).

Answer

1) \(\lim_{n\to\infty}b_n=\infty\). 2) \(n_0=42\). 3) \(n_1=335\).
53444712
The graph shows \(f(x) = \sin(x)\), with \(x\) measured in radians. Secant line \(s\) passes through the origin \(O(0, 0)\) and \(P(x, \sin(x))\). The dashed line \(t\) is tangent at the origin. 1. Write an expression for the slope \(m_s\) of secant line \(OP\) in terms of \(x\). 2. Use the graph to find the slope of tangent line \(t\). 3. Explain what happens to the secant line as \(x\to 0\), and use the graph to determine \(\lim_{x\to 0}\frac{\sin(x)}{x}\).
Figure for problem 534447

Hints

- Use the slope formula between \(O\) and \(P\). - Read the tangent slope from two grid points. - Imagine moving \(P\) closer to the origin. - Connect the limiting secant slope to the tangent slope.

Solution

1. The secant slope is \(m_s = \frac{\sin(x)-\sin(0)}{x-0} = \frac{\sin(x)}{x}\). 2. The tangent line passes through \((0, 0)\) and \((1, 1)\), so its slope is \(1\). 3. As \(x\to 0\), point \(P\) approaches \(O\), and the secant line approaches the tangent line. 4. Therefore, the secant slopes approach the tangent slope, so \(\lim_{x\to 0}\frac{\sin(x)}{x} = 1\).

Answer

1. \(m_s = \frac{\sin(x)}{x}\) 2. \(1\) 3. The secant approaches the tangent, and \(\lim_{x\to 0}\frac{\sin(x)}{x} = 1\).

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