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Properties of logarithms

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52760112
A student claims: “The equation \(\ln(x^2)=2\ln(x)\) is valid for every \(x\in\mathbb{R}\setminus\{0\}\) because the exponent can be moved in front of the logarithm.” Evaluate the claim by comparing the domains of the two sides.

Hints

- When is a natural logarithm defined over the real numbers? - Test a negative value of \(x\). - Compare the domains of both sides before applying a logarithm property. - What happens to a negative number when it is squared?

Solution

1. The left side, \(\ln(x^2)\), is defined when \(x^2>0\), which means \(x\ne0\). 2. The right side, \(2\ln(x)\), is defined only when \(x>0\). 3. The domains are not the same. For example, when \(x=-2\), \(\ln(x^2)=\ln(4)\) is defined, but \(2\ln(-2)\) is not real. 4. Therefore, the student's statement is false. The identity \(\ln(x^2)=2\ln(x)\) is valid only for \(x>0\). For all \(x\ne0\), the correct identity is \(\ln(x^2)=2\ln|x|\).

Answer

The claim is false. The left side is defined for \(x\ne0\), while the right side is defined only for \(x>0\).
52987512
Condense the expression into a single natural logarithm and simplify as far as possible: \(3\ln(x)+\frac{1}{2}\ln(y)-\ln(z^2)\).

Hints

- Move each coefficient inside its logarithm as an exponent. - A sum of logarithms becomes a product. - A difference of logarithms becomes a quotient.

Solution

1. Use the power property: \(3\ln(x)=\ln(x^3)\) and \(\frac{1}{2}\ln(y)=\ln(\sqrt{y})\). 2. Use the product and quotient properties: \(\ln(x^3)+\ln(\sqrt{y})-\ln(z^2)=\ln\!\left(\frac{x^3\sqrt{y}}{z^2}\right)\).

Answer

\(\ln\!\left(\frac{x^3\sqrt{y}}{z^2}\right)\)
52987612
Expand the logarithm as a sum and difference of simpler logarithms. Simplify all expressions involving \(e\). Assume \(a>0\), \(b>0\), and \(c>0\). \(\ln\!\left(\frac{\sqrt{e}\,a^4}{bc^2}\right)\)

Hints

- Separate the quotient first. - Expand the products in the numerator and denominator. - Move exponents in front of logarithms. - Rewrite \(\sqrt{e}\) as a power of \(e\).

Solution

1. Apply the quotient and product properties: \(\ln(\sqrt{e})+\ln(a^4)-\ln(b)-\ln(c^2)\). 2. Apply the power property and simplify \(\ln(\sqrt{e})=\ln(e^{1/2})=\frac{1}{2}\). 3. The result is \(\frac{1}{2}+4\ln(a)-\ln(b)-2\ln(c)\).

Answer

\(\frac{1}{2}+4\ln(a)-\ln(b)-2\ln(c)\)
52988112
Prove that \(\ln(\sqrt{x})=\frac{1}{2}\ln(x)\) for every \(x>0\), using the product property of logarithms.

Hints

- Write \(x\) as a product of two equal square-root factors. - Apply the product property of logarithms. - Combine the two identical terms.

Solution

1. Since \(x>0\), write \(x=\sqrt{x}\sqrt{x}\). 2. Take natural logarithms: \(\ln(x)=\ln(\sqrt{x}\sqrt{x})\). 3. By the product property, \(\ln(x)=\ln(\sqrt{x})+\ln(\sqrt{x})=2\ln(\sqrt{x})\). 4. Divide by \(2\) to obtain \(\ln(\sqrt{x})=\frac{1}{2}\ln(x)\).

Answer

\(\ln(x)=2\ln(\sqrt{x})\), so \(\ln(\sqrt{x})=\frac{1}{2}\ln(x)\).
53266712
The graph shows \(f(x)=\ln(x)\) together with two graphs, \(g\) and \(h\), obtained from \(f\) by translations only. Find formulas for \(g\) and \(h\), and describe each translation.
Figure for problem 532667

Hints

- Compare the vertical asymptotes. - Track the location of the \(x\)-intercept \((1, 0)\). - A horizontal shift changes the input; a vertical shift changes the output.

Solution

1. The graph of \(f\) has vertical asymptote \(x=0\) and \(x\)-intercept \((1, 0)\). 2. The graph of \(g\) has vertical asymptote \(x=2\) and \(x\)-intercept \((3, 0)\). It is the graph of \(f\) shifted right \(2\) units, so \(g(x)=\ln(x-2)\). 3. The graph of \(h\) has the same vertical asymptote as \(f\), and the point \((1, 0)\) has moved to \((1, 2)\). It is the graph of \(f\) shifted up \(2\) units, so \(h(x)=\ln(x)+2\).

Answer

\(g(x)=\ln(x-2)\), a shift right \(2\) units \(h(x)=\ln(x)+2\), a shift up \(2\) units
52185712
Let \(f(x)=\ln(x)\) for \(x>0\). The graph of \(g\) is the reflection of the graph of \(f\) across the vertical line \(x=5\). Find an equation for \(g\) and the coordinates of the reflected image \(P'\) of \(P(1, 0)\). Then verify algebraically that \(P'\) lies on the graph of \(g\).

Hints

- Compare the horizontal distance from a point to the reflection line before and after reflection. - How does reflection across a vertical line change the x-coordinate? - Does the y-coordinate change under this reflection? - Substitute the reflected point into the new function.

Solution

1. Reflection across a vertical line \(x=a\) maps an x-coordinate to \(2a-x\). For \(a=5\), replace \(x\) with \(10-x\). 2. Therefore, \(g(x)=f(10-x)=\ln(10-x)\), with domain \(x<10\). 3. The reflected x-coordinate of \(P(1, 0)\) is \(2\cdot5-1=9\), while the y-coordinate stays unchanged. Thus, \(P'=(9, 0)\). 4. Since \(g(9)=\ln(10-9)=\ln(1)=0\), the point \(P'\) lies on the graph of \(g\).

Answer

\(g(x)=\ln(10-x)\), with domain \((-\infty, 10)\); \(P'=(9, 0)\). Since \(g(9)=0\), the point lies on the graph.
52615712
Determine the number of digits in the decimal representation of \(6^{60}\). Use \(\log(2)\approx0.3010\) and \(\log(3)\approx0.4771\).

Hints

- How is the number of digits of a positive integer related to its common logarithm? - Which logarithm properties let you rewrite \(\log(6^{60})\)? - What does the integer part of a common logarithm tell you about the size of a number?

Solution

1. Use the power property: \(\log(6^{60})=60\log(6)\). 2. Use the product property: \(\log(6)=\log(2\cdot3)=\log(2)+\log(3)\). 3. Substitute the given approximations: \(\log(6)\approx0.3010+0.4771=0.7781\). 4. Then \(\log(6^{60})\approx60\cdot0.7781=46.686\). 5. For a positive integer \(N\), the number of decimal digits is \(\lfloor\log(N)\rfloor+1\). Therefore, \(6^{60}\) has \(46+1=47\) digits.

Answer

\(6^{60}\) has \(47\) digits.
52615812
A computer program can store whole numbers with at most \(100\) decimal digits. Find the greatest natural number \(n\) for which \(2^n\) can still be stored. Use \(\log(2)\approx0.30103\).

Hints

- What condition must the common logarithm satisfy for a number to have at most \(100\) digits? - Write an inequality involving \(n\). - Use the power property of logarithms.

Solution

1. A positive integer has at most \(100\) digits when its common logarithm is less than \(100\), so \(\log(2^n)<100\). 2. Apply the power property: \(n\log(2)<100\). 3. Then \(n<\frac{100}{\log(2)}\approx\frac{100}{0.30103}\approx332.19\). 4. Since \(n\) is a natural number, the greatest possible value is \(n=332\). 5. Check: \(\log(2^{332})\approx99.94\), so \(2^{332}\) has \(100\) digits, while \(\log(2^{333})\approx100.24\), so \(2^{333}\) has \(101\) digits.

Answer

\(n=332\)
52623712
Simplify each expression completely. a) \(\ln\!\left(\sqrt[3]{e}\right)\) b) \(e^{3\ln(2)-\ln(4)}\) c) \(\ln\!\left(\frac{e^x}{e^{x-2}}\right)\) d) \(e^{\ln(x^2)}e^{-\ln(x)}\), for \(x>0\)

Hints

- Recall that the natural logarithm and the exponential function are inverses. - Rewrite radicals as rational exponents. - Apply logarithm rules for products and quotients. - Use exponent rules for powers with the same base.

Solution

1. \(\sqrt[3]{e}=e^{1/3}\), so \(\ln(e^{1/3})=\frac{1}{3}\). 2. \(3\ln(2)=\ln(2^3)=\ln(8)\). Therefore, \(3\ln(2)-\ln(4)=\ln\!\left(\frac{8}{4}\right)=\ln(2)\), and \(e^{\ln(2)}=2\). 3. \(\frac{e^x}{e^{x-2}}=e^{x-(x-2)}=e^2\), so \(\ln(e^2)=2\). 4. Since \(x>0\), \(e^{\ln(x^2)}=x^2\) and \(e^{-\ln(x)}=\frac{1}{x}\). Their product is \(x\).

Answer

a) \(\frac{1}{3}\) b) \(2\) c) \(2\) d) \(x\)
52623812
Let \(f(x)=\ln(e^3x)-\ln(x)\), where \(x>0\). a) Use logarithm properties to show that \(f\) is constant, and state its value. b) Solve \(e^{2\ln(x)+1}=e^3\).

Hints

- Expand the logarithm of a product. - Equal powers with base \(e\) have equal exponents. - Apply the exponential function after isolating \(\ln(x)\).

Solution

1. Use the product property: \(\ln(e^3x)=\ln(e^3)+\ln(x)=3+\ln(x)\). 2. Therefore, \(f(x)=3+\ln(x)-\ln(x)=3\) for every \(x>0\). 3. Since the exponential expressions in part b have the same base, set the exponents equal: \(2\ln(x)+1=3\). 4. Then \(2\ln(x)=2\), so \(\ln(x)=1\) and \(x=e\).

Answer

a) \(f(x)=3\) b) \(x=e\)
52739812
The function \(f\) is defined by \(f(x) = \frac{(x - 5)\ln(x)}{x^2 - 1}\). State the domain of \(f\) and find all x-intercepts.

Hints

- What restriction does \(\ln(x)\) place on \(x\)? - Which values make the denominator equal to \(0\)? - A value that makes both numerator and denominator zero is still excluded from the original function's domain. - Use the zero-product property on the numerator.

Solution

1. The natural logarithm requires \(x > 0\). The denominator must also be nonzero: \(x^2 - 1 \ne 0\), so \(x \ne -1\) and \(x \ne 1\). Because \(-1\) is already excluded by \(x > 0\), the domain is \((0, 1) \cup (1, \infty)\). 2. An x-intercept can occur when the numerator is zero: \((x - 5)\ln(x) = 0\). Thus, \(x - 5 = 0\) or \(\ln(x) = 0\), giving candidates \(x = 5\) and \(x = 1\). 3. The value \(x = 1\) is not in the domain because the denominator is zero there. The only x-intercept is \((5, 0)\).

Answer

The domain is \((0, 1) \cup (1, \infty)\), and the only x-intercept is \((5, 0)\).
52741812
Find the domain and all x-intercepts of \(g(x) = \frac{x^2 - 4}{\ln(x + 3)}\).

Hints

- State the condition that makes the logarithm defined. - Also require the entire denominator to be nonzero. - Find the zeros of the numerator. - Check each candidate zero against the domain.

Solution

1. The logarithm requires \(x + 3 > 0\), so \(x > -3\). 2. The denominator must be nonzero. Since \(\ln(x + 3) = 0\) when \(x + 3 = 1\), exclude \(x = -2\). Therefore, the domain is \((-3, -2) \cup (-2, \infty)\). 3. For an x-intercept, the numerator must be zero: \(x^2 - 4 = 0\). Thus, \(x = -2\) or \(x = 2\). 4. The value \(x = -2\) is not in the domain, while \(x = 2\) is. Therefore, the only x-intercept is \((2, 0)\).

Answer

Domain: \((-3, -2) \cup (-2, \infty)\) X-intercept: \((2, 0)\)
52758512
Use \(\ln(2)\approx0.69\) and \(\ln(5)\approx1.61\) with logarithm properties to approximate each value. a) \(\ln(10)\) b) \(\ln(2.5)\) c) \(\ln(80)\) d) \(\ln(0.4)\) e) \(\ln(\sqrt{5})\)

Hints

- Express each logarithm input as a product or quotient involving \(2\) and \(5\). - Rewrite decimals such as \(2.5\) and \(0.4\) as fractions. - Express a square root as a power. - Use the power property to move an exponent in front of a logarithm.

Solution

1. \(\ln(10)=\ln(2\cdot5)=\ln(2)+\ln(5)\approx0.69+1.61=2.30\). 2. \(\ln(2.5)=\ln\!\left(\frac{5}{2}\right)=\ln(5)-\ln(2)\approx1.61-0.69=0.92\). 3. \(\ln(80)=\ln(2^4\cdot5)=4\ln(2)+\ln(5)\approx4(0.69)+1.61=4.37\). 4. \(\ln(0.4)=\ln\!\left(\frac{2}{5}\right)=\ln(2)-\ln(5)\approx-0.92\). 5. \(\ln(\sqrt{5})=\frac{1}{2}\ln(5)\approx0.805\).

Answer

a) \(\approx2.30\) b) \(\approx0.92\) c) \(\approx4.37\) d) \(\approx-0.92\) e) \(\approx0.805\)
52760212
Consider the equation \(\ln\!\left(\frac{x-2}{x-5}\right)=\ln(x-2)-\ln(x-5)\). Is this equation valid for every x-value for which the left side is defined? Justify your answer by analyzing the domains.

Hints

- Determine when a quotient is positive by comparing the signs of its numerator and denominator. - Each logarithm on the right must have a positive input. - Compare the two domain conditions. - Look for x-values where the left side exists but the right side does not.

Solution

1. For the left side, require \(\frac{x-2}{x-5}>0\). The numerator and denominator have the same sign when \(x<2\) or \(x>5\). Thus, the left-side domain is \((-\infty, 2)\cup(5, \infty)\). 2. For the right side, both logarithm inputs must be positive: \(x-2>0\) and \(x-5>0\). Together, these conditions give \(x>5\). 3. Therefore, the equation is valid only for \(x>5\). When \(x<2\), the quotient on the left is positive, but each individual logarithm on the right has a negative input and is not real.

Answer

No. The left side is defined for \(x<2\) or \(x>5\), but the right side is defined only for \(x>5\).
52762912
Let \(f(x)=\ln(x^2+e)\), with domain \(\mathbb{R}\). Find the range of \(f\) and justify your answer.

Hints

- Find the minimum possible value of \(x^2+e\). - Is the natural logarithm increasing or decreasing? - What happens as \(|x|\) becomes large? - Recall the value of \(\ln(e)\).

Solution

1. Since \(x^2\geq0\), the logarithm input satisfies \(x^2+e\geq e\). 2. The natural logarithm is strictly increasing, so \(f(x)\geq\ln(e)=1\). 3. Equality occurs at \(x=0\). As \(|x|\to\infty\), \(x^2+e\to\infty\), so \(f(x)\to\infty\). 4. Because \(f\) is continuous, it takes every value from \(1\) upward. Thus, its range is \([1, \infty)\).

Answer

\([1, \infty)\)
52763012
Let \(g(x)=\ln\!\left(\frac{e^2}{x^4+1}\right)\), with domain \(\mathbb{R}\). Justify that the range of \(g\) is \((-\infty, 2]\).

Hints

- Find the minimum possible value of the denominator. - Determine the resulting range of the logarithm input. - What happens to \(\ln(u)\) as \(u\to0^+\)? - Evaluate the function at \(x=0\).

Solution

1. Since \(x^4+1\geq1\), \(0<\frac{e^2}{x^4+1}\leq e^2\). The maximum input to the logarithm is \(e^2\), attained at \(x=0\). 2. Because the natural logarithm is strictly increasing, the maximum value is \(g(0)=\ln(e^2)=2\). 3. As \(|x|\to\infty\), the fraction \(\frac{e^2}{x^4+1}\to0^+\), so \(g(x)\to-\infty\). 4. The function is continuous, so it takes every value less than or equal to \(2\). Therefore, the range is \((-\infty, 2]\).

Answer

\((-\infty, 2]\)
52765912
Find the domain of \(f(x)=\ln\!\left(1+\frac{4}{x-2}\right)\).

Hints

- The input of a logarithm must be positive. - Combine the expression inside the logarithm into one fraction. - Identify where the numerator is zero and where the denominator is zero. - Use a sign chart to determine where the fraction is positive.

Solution

1. The logarithm input must be positive: \(1+\frac{4}{x-2}>0\). 2. Combine the terms: \(1+\frac{4}{x-2}=\frac{x+2}{x-2}\). 3. The critical values are \(x=-2\), where the numerator is zero, and \(x=2\), where the expression is undefined. 4. A sign analysis shows that \(\frac{x+2}{x-2}>0\) when \(x<-2\) or \(x>2\). 5. Therefore, the domain is \((-\infty, -2)\cup(2, \infty)\).

Answer

\((-\infty, -2)\cup(2, \infty)\)
52767612
Let \(f(x)=\ln(x)\) and \(p(x)=\ln((x-2)^2)\). a) State the domain of \(p\) and the equation of its vertical asymptote. b) Describe the transformations that produce the graph of \(p\) from the graph of \(f\). c) Find the \(x\)-intercepts of the graph of \(p\).

Hints

- Determine when \((x-2)^2\) is positive. - Use \(\ln(a^2)=2\ln|a|\). - Read the horizontal shift from \(x-2\). - A logarithm equals \(0\) when its input equals \(1\).

Solution

1. The logarithm input must be positive. Since \((x-2)^2>0\) for every \(x\neq2\), the domain is \(\mathbb{R}\setminus\{2\}\). The vertical asymptote is \(x=2\). 2. Rewrite \(p(x)=2\ln|x-2|\). Starting with \(y=\ln(x)\), reflect its right-hand branch across the \(y\)-axis to create \(y=\ln|x|\), stretch vertically by a factor of \(2\), and shift right \(2\) units. 3. For the \(x\)-intercepts, solve \(\ln((x-2)^2)=0\). Then \((x-2)^2=1\), so \(x=1\) or \(x=3\).

Answer

a) Domain: \(\mathbb{R}\setminus\{2\}\); vertical asymptote: \(x=2\) b) Reflect the right-hand branch across the \(y\)-axis, stretch vertically by a factor of \(2\), and shift right \(2\) units. c) \((1, 0)\) and \((3, 0)\)
52984112
The natural exponential function \(f(x)=e^x\) is a special case of \(g(x)=b^x\), where \(b>0\) and \(b\ne1\). Every such function can be written in the form \(e^{kx}\). 1. Express \(k\) in terms of \(b\). 2. Determine the values of \(b\) for which \(g\) is strictly decreasing. Explain the result using both the base \(b\) and the sign of \(k\). 3. State the range of \(g\), and explain why its graph never crosses or touches the x-axis.

Hints

- Use the inverse relationship between \(e^x\) and \(\ln(x)\). - For which bases do powers decrease as the exponent increases? - What is the sign of \(\ln(b)\) when \(0<b<1\)? - Can a positive base raised to a real power equal zero?

Solution

1. Since \(b=e^{\ln(b)}\), \(b^x=\left(e^{\ln(b)}\right)^x=e^{x\ln(b)}\). Therefore, \(k=\ln(b)\). 2. The function \(b^x\) is strictly decreasing when \(0<b<1\). On this interval, \(\ln(b)<0\), so the equivalent condition is \(k<0\). 3. For every allowed base and every real \(x\), \(b^x>0\). Therefore, the range is \((0,\infty)\). The graph may approach the x-axis as an asymptote, but it cannot reach \(y=0\).

Answer

1. \(k=\ln(b)\) 2. \(g\) is strictly decreasing for \(0<b<1\), which is equivalent to \(k<0\). 3. The range is \((0,\infty)\), and \(b^x\) is never \(0\).
52985712
The natural logarithm is available directly on most scientific calculators. Other logarithms can be evaluated by changing the base. a) Starting with \(y=\log_b(x)\), prove the change-of-base formula \(\log_b(x)=\frac{\ln(x)}{\ln(b)}\) for \(x>0\), \(b>0\), and \(b\neq1\). b) Use natural logarithms to evaluate \(\log_5(125)\), \(\log_2(10)\), and \(\log_{0.5}(4)\). Round \(\log_2(10)\) to four decimal places. c) Explain without additional numerical calculation why \(\ln(5)\log_5(e)=1\).

Hints

- Rewrite a logarithmic equation in exponential form. - Take natural logarithms of both sides and use the power property. - Express exact logarithm inputs as powers of their bases when possible. - Recall that \(\ln(e)=1\).

Solution

1. If \(y=\log_b(x)\), then \(b^y=x\). Taking natural logarithms gives \(\ln(b^y)=\ln(x)\), so \(y\ln(b)=\ln(x)\). Therefore, \(y=\frac{\ln(x)}{\ln(b)}\). 2. \(\log_5(125)=\frac{\ln(125)}{\ln(5)}=3\). 3. \(\log_2(10)=\frac{\ln(10)}{\ln(2)}\approx3.3219\). 4. \(\log_{0.5}(4)=\frac{\ln(4)}{\ln(0.5)}=-2\). 5. By change of base, \(\log_5(e)=\frac{\ln(e)}{\ln(5)}=\frac{1}{\ln(5)}\). Therefore, \(\ln(5)\log_5(e)=1\).

Answer

a) \(\log_b(x)=\frac{\ln(x)}{\ln(b)}\) b) \(\log_5(125)=3\); \(\log_2(10)\approx3.3219\); \(\log_{0.5}(4)=-2\) c) \(\log_5(e)=\frac{1}{\ln(5)}\), so the product is \(1\).
52985812
Use natural logarithms and logarithm properties to complete each task. a) Solve \(5^x=20\) exactly, and give an approximation rounded to three decimal places. b) Prove that \(\log_a(x)=\frac{1}{\log_x(a)}\) for \(x>0\), \(x\neq1\), \(a>0\), and \(a\neq1\). c) Evaluate \(\ln(e^3)+\log_{e^2}(e)\) exactly.

Hints

- Take a logarithm when the variable is in an exponent. - Apply the change-of-base formula to both logarithms in part b. - Use the inverse relationship between logarithms and exponential functions. - Determine the exponent that takes \(e^2\) to \(e\).

Solution

1. Taking natural logarithms of \(5^x=20\) gives \(x\ln(5)=\ln(20)\). Thus, \(x=\frac{\ln(20)}{\ln(5)}\approx1.861\). 2. By change of base, \(\log_a(x)=\frac{\ln(x)}{\ln(a)}\) and \(\log_x(a)=\frac{\ln(a)}{\ln(x)}\). Therefore, \(\frac{1}{\log_x(a)}=\frac{\ln(x)}{\ln(a)}=\log_a(x)\). 3. Since \(\ln(e^3)=3\) and \(\log_{e^2}(e)=\frac{1}{2}\), the value is \(3+\frac{1}{2}=\frac{7}{2}\).

Answer

a) \(x=\frac{\ln(20)}{\ln(5)}\approx1.861\) b) \(\log_a(x)=\frac{1}{\log_x(a)}\) c) \(\frac{7}{2}\)
52985912
Consider \(f(x)=\ln(x)\) and \(g(x)=\log_{1/e}(x)\), where \(x>0\). 1. Use the change-of-base formula to show that \(g(x)=-\ln(x)\). 2. For each function, state the interval on which its values are negative. 3. Use \(f\) and \(g\) to explain how the base of a logarithmic function determines whether the function is increasing or decreasing.

Hints

- Apply the change-of-base formula with base \(e^{-1}\). - Recall that every basic logarithmic graph passes through \((1, 0)\). - Compare the graphs of \(y=\ln(x)\) and \(y=-\ln(x)\). - Relate the base interval to the monotonicity of the corresponding exponential function.

Solution

1. By change of base, \(g(x)=\frac{\ln(x)}{\ln(e^{-1})}=\frac{\ln(x)}{-1}=-\ln(x)\). 2. Since \(\ln(x)<0\) for \(0<x<1\), \(f\) is negative on \((0, 1)\). Since \(g(x)=-\ln(x)\), \(g\) is negative when \(x>1\). 3. A logarithmic function with base greater than \(1\) is strictly increasing. A logarithmic function with base between \(0\) and \(1\) is strictly decreasing. Here, \(e>1\), so \(f\) increases, while \(1/e<1\), so \(g\) decreases.

Answer

1. \(g(x)=-\ln(x)\) 2. \(f(x)<0\) on \((0, 1)\); \(g(x)<0\) on \((1, \infty)\) 3. Bases greater than \(1\) produce increasing logarithmic functions; bases between \(0\) and \(1\) produce decreasing logarithmic functions.
52987412
Let \(g(x)=\log_2(x)\). a) Explain why doubling \(x\) increases \(g(x)\) by exactly \(1\). b) Use this property to show that \(g\) is unbounded above. c) Find the inputs where \(g(x)=20\) and \(g(x)=30\).

Hints

- Apply the product property of logarithms. - Consider what happens after doubling the input many times. - Rewrite each logarithmic equation in exponential form.

Solution

1. By the product property, \(g(2x)=\log_2(2x)=\log_2(2)+\log_2(x)=1+g(x)\). 2. Repeatedly doubling the input adds \(1\) to the output each time. Therefore, the outputs can exceed any fixed bound, so \(g\) is unbounded above. 3. If \(\log_2(x)=20\), then \(x=2^{20}=1{,}048{,}576\). 4. If \(\log_2(x)=30\), then \(x=2^{30}=1{,}073{,}741{,}824\).

Answer

a) \(g(2x)=g(x)+1\) b) Repeated doubling makes the outputs increase without bound. c) \(x=1{,}048{,}576\) and \(x=1{,}073{,}741{,}824\), respectively.
52988412
For each positive integer \(n\), let \(g_n(x)=(\ln(x))^n\), where \(x>0\). Find all points that lie on every graph in this family. Justify your answer.

Hints

- At a common point, the output must not change with \(n\). - Set \(u=\ln(x)\) and compare the cases \(n=1\) and \(n=2\). - Convert the resulting values of \(u\) back to values of \(x\).

Solution

1. Let \(u=\ln(x)\). A common point requires \(u^n\) to have the same value for every positive integer \(n\). 2. In particular, \(u=u^2\), so \(u(u-1)=0\). Thus, \(u=0\) or \(u=1\). 3. If \(u=0\), then \(x=1\) and every output is \(0\), giving \((1, 0)\). 4. If \(u=1\), then \(x=e\) and every output is \(1\), giving \((e, 1)\). 5. Both values work for all \(n\), and the equation \(u=u^2\) shows there are no others.

Answer

\((1, 0)\) and \((e, 1)\)
52991212
Consider \(f(x)=4^x\) and \(g(x)=e^{1.4x}\). Determine which function grows faster for \(x>0\). Justify your answer by rewriting \(f\) with base \(e\) and comparing the coefficients of \(x\) in the exponents.

Hints

- Rewrite both functions using the same base. - Compare \(\ln(4)\) with \(1.4\). - How does a larger positive coefficient in the exponent affect exponential growth?

Solution

1. Rewrite the base: \(4=e^{\ln(4)}\). Thus, \(f(x)=e^{x\ln(4)}\). 2. Since \(\ln(4)\approx1.3863\) and \(1.4>1.3863\), the exponent of \(g\) grows faster than the exponent of \(f\) for \(x>0\). 3. Therefore, \(g(x)>f(x)\) for every \(x>0\). Also, \(g'(x)=1.4e^{1.4x}\) is greater than \(f'(x)=\ln(4)e^{x\ln(4)}\) for \(x>0\), so \(g\) has the greater growth rate.

Answer

\(g(x)=e^{1.4x}\) grows faster because \(f(x)=e^{x\ln(4)}\), \(\ln(4)\approx1.3863\), and \(1.4>1.3863\).
53452712
The coordinate plane shows the graph of a linear function \(f\). Define \(g(x)=\ln(f(x))\). a) Find the domain of \(g\). b) Describe the behavior of \(g\) at both ends of its domain. c) Explain whether \(g\) is increasing or decreasing on its domain.
Figure for problem 534527

Hints

- Identify where the graph of \(f\) lies above the \(x\)-axis. - Recall the behavior of \(\ln(u)\) as \(u\to0^+\) and as \(u\to\infty\). - Consider the composition of two increasing functions.

Solution

1. From the graph, \(f(x)=0.5x+1\). The logarithm requires \(f(x)>0\), so \(0.5x+1>0\), which gives \(x>-2\). Thus, the domain is \((-2, \infty)\). 2. As \(x\to-2^+\), \(f(x)\to0^+\), so \(g(x)\to-\infty\). As \(x\to\infty\), \(f(x)\to\infty\), so \(g(x)\to\infty\). 3. The linear function \(f\) is strictly increasing, and \(\ln(x)\) is strictly increasing. Therefore, their composition \(g\) is strictly increasing on \((-2, \infty)\).

Answer

a) \((-2, \infty)\) b) \(\lim_{x\to-2^+}g(x)=-\infty\) and \(\lim_{x\to\infty}g(x)=\infty\) c) \(g\) is strictly increasing.
53458512
The graph shows \(f(x)=\ln(x)\) together with two graphs, \(g\) and \(h\). a) Find formulas for \(g\) and \(h\). b) Describe how each graph is obtained from the graph of \(f\).
Figure for problem 534585

Hints

- Compare the outputs at \(x=e\). - Check whether the graphs share the same \(x\)-intercept. - Look for a constant scale factor or a sign change.

Solution

1. All three graphs pass through \((1, 0)\), so there is no translation. 2. At \(x=e\), \(f(e)=1\), while the graph of \(g\) has value \(2\). Thus, \(g(x)=2\ln(x)\), a vertical stretch by a factor of \(2\). 3. The graph of \(h\) has outputs opposite those of \(f\). Thus, \(h(x)=-\ln(x)\), a reflection across the \(x\)-axis.

Answer

a) \(g(x)=2\ln(x)\) and \(h(x)=-\ln(x)\) b) \(g\) is a vertical stretch by a factor of \(2\); \(h\) is a reflection across the \(x\)-axis.
53459812
The graph shows a rational function \(f\). For the composite function \(g(x)=\ln(f(x))\), find the domain and describe the monotonicity on each interval of the domain.
Figure for problem 534598

Hints

- Find where the graph of \(f\) is above the \(x\)-axis. - Read the monotonicity of \(f\) on those intervals. - An increasing outer function preserves the monotonicity of the inner function.

Solution

1. The logarithm requires \(f(x)>0\). From the graph, \(f(x)>0\) for \(x<-1\) and \(x>1\). Thus, the domain is \((-\infty, -1)\cup(1, \infty)\). 2. The graph shows that \(f\) is strictly decreasing on both \((-\infty, -1)\) and \((1, \infty)\). 3. Since the natural logarithm is strictly increasing, composing with it preserves the direction of monotonicity. Therefore, \(g\) is strictly decreasing on each domain interval.

Answer

Domain: \((-\infty, -1)\cup(1, \infty)\) The function is strictly decreasing on \((-\infty, -1)\) and on \((1, \infty)\).
52617412
Investigate how logarithms change with the base. a) Find the constant \(k\) such that \(\ln(x) = k\log(x)\) for every \(x > 0\). Round \(k\) to four decimal places. b) Consider the claim: “If the base of a logarithm is doubled, the logarithm’s value is cut in half.” Test the claim using \(\log_2(16)\) and \(\log_4(16)\). Then give a counterexample showing that the claim is not true for all bases \(b > 1\) and arguments \(a > 0\).

Hints

- Use the change-of-base formula in part a. - Choose familiar powers when testing the claim. - Compare doubling a base with squaring a base. - Determine what change to the denominator in the change-of-base formula would halve the result.

Solution

1. a) By change of base, \(\ln(x) = \frac{\log(x)}{\log(e)}\). Thus, \(k = \frac{1}{\log(e)} = \ln(10) \approx 2.3026\). 2. b) \(\log_2(16) = 4\) and \(\log_4(16) = 2\), so the claim happens to be true in this case. 3. For a counterexample, \(\log_3(9) = 2\), but doubling the base gives \(\log_6(9) = \frac{\ln(9)}{\ln(6)} \approx 1.2263\), not \(1\). 4. In general, squaring the base halves the logarithm because \(\log_{b^2}(a) = \frac{\ln(a)}{\ln(b^2)} = \frac{1}{2}\log_b(a)\). Doubling the base does not generally have that effect.

Answer

a) \(k \approx 2.3026\) b) The claim works for \(\log_2(16) = 4\) and \(\log_4(16) = 2\), but it is false in general. For example, \(\log_3(9) = 2\), while \(\log_6(9) \approx 1.2263 \ne 1\).
52634212
Consider \(f(x)=\ln\!\left(\frac{e}{x^2}\right)\). a) State the largest possible real domain of \(f\). b) Use logarithm properties to show that \(f(x)=1-2\ln|x|\). c) Describe how the graph of \(f\) can be obtained from the graph of \(y=\ln(x)\). Address symmetry and the number of branches.

Hints

- Require the logarithm input to be positive. - Use the quotient and power properties of logarithms. - What does \(f(-x)=f(x)\) imply about the graph? - Separate the effects of the coefficient, negative sign, and vertical shift.

Solution

1. The expression \(\frac{e}{x^2}\) is defined and positive whenever \(x\ne0\). Thus, the domain is \(\mathbb{R}\setminus\{0\}\). 2. Apply the quotient property: \(f(x)=\ln(e)-\ln(x^2)=1-\ln(x^2)\). For \(x\ne0\), \(\ln(x^2)=2\ln|x|\), so \(f(x)=1-2\ln|x|\). 3. Begin with the right-hand branch of \(y=\ln(x)\). Reflect it across the y-axis to create the left-hand branch, producing \(y=\ln|x|\). 4. Stretch vertically by a factor of \(2\), reflect across the x-axis, and shift up \(1\) unit. Because \(f(-x)=f(x)\), the final graph is symmetric about the y-axis and has two branches.

Answer

a) \(\mathbb{R}\setminus\{0\}\) b) \(f(x)=1-2\ln|x|\) c) Create \(\ln|x|\) by reflecting the right branch across the y-axis, stretch vertically by \(2\), reflect across the x-axis, and shift up \(1\) unit. The graph has two branches and y-axis symmetry.
52986012
For \(a>0\), \(a\neq1\), the value \(\log_a(x)\) is positive exactly when the base \(a\) and the argument \(x\) are on the same side of \(1\): either both are greater than \(1\), or both are between \(0\) and \(1\). 1. Prove this statement using \(a^y=x\) and the monotonicity of exponential functions for the cases \(a>1\) and \(0<a<1\). 2. Without calculating its value, determine the sign of \(\log_{0.2}(5)\) and justify your answer.

Hints

- Set \(y=\log_a(x)\) and rewrite in exponential form. - Compare \(a^y\) with \(a^0\) when \(y>0\). - Treat increasing and decreasing exponential functions separately. - Locate \(0.2\) and \(5\) relative to \(1\).

Solution

1. Let \(y=\log_a(x)\), so \(a^y=x\). 2. If \(a>1\), the function \(a^y\) is increasing. Thus, \(y>0\) exactly when \(a^y>a^0=1\), which means \(x>1\). 3. If \(0<a<1\), the function \(a^y\) is decreasing. Thus, \(y>0\) exactly when \(a^y<a^0=1\), which means \(0<x<1\). 4. Therefore, \(\log_a(x)>0\) exactly when \(a\) and \(x\) are on the same side of \(1\). 5. For \(\log_{0.2}(5)\), the base is less than \(1\) and the argument is greater than \(1\), so the value is negative.

Answer

1. The logarithm is positive exactly when the base and argument are both greater than \(1\) or both lie between \(0\) and \(1\). 2. \(\log_{0.2}(5)<0\)
53458912
Match each graph with its function. Justify each match. \(h_1(x)=\ln(x+3)\) \(h_2(x)=\ln(x^2+1)\) \(h_3(x)=x-\ln(x)\) \(h_4(x)=\frac{1}{\ln(x)}\)
Figure for problem 534589

Hints

- Compare domains and vertical asymptotes. - Check each function for symmetry. - Locate intercepts and extrema. - Identify where a denominator becomes zero.

Solution

1. Graph a is \(h_1\). Its domain is \(x>-3\), and its \(x\)-intercept is \((-2, 0)\). 2. Graph b is \(h_2\). It is even, so it is symmetric about the \(y\)-axis, and its minimum is \((0, 0)\). 3. Graph c is \(h_3\). Its domain is \(x>0\). Since \(h_3'(x)=1-\frac{1}{x}\), it has a critical point at \(x=1\), where its minimum value is \(1\). 4. Graph d is \(h_4\). It is undefined at \(x=1\), producing a vertical asymptote there. As \(x\to0^+\), its values approach \(0\) from below.

Answer

a) \(h_1(x)=\ln(x+3)\) b) \(h_2(x)=\ln(x^2+1)\) c) \(h_3(x)=x-\ln(x)\) d) \(h_4(x)=\frac{1}{\ln(x)}\)

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