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Interpret the derivative in context

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52228812
The following functions model measurements of round solids. Differentiate each function with respect to the stated variable, and explain the geometric meaning of the derivative. a) \(M(h)=2\pi rh\), the lateral surface area of a cylinder with fixed radius \(r\) and variable height \(h\) b) \(S(r)=4\pi r^2\), the surface area of a sphere with variable radius \(r\)

Hints

- Identify the variable of differentiation and treat the other letters as constants. - Which circle measurement is represented by \(2\pi r\)? - Think about how a cylinder's lateral area changes when only its height changes. - For the sphere, interpret the derivative as a rate for an extremely small change in radius.

Solution

1. For part a), treat \(r\) as constant: \(M'(h)=2\pi r\). This derivative equals the circumference of the circular base and gives the exact increase in lateral surface area per unit increase in height. 2. For part b), \(S'(r)=8\pi r\). This is the instantaneous rate of change of surface area with respect to radius, in square units per unit of radius. It is also four times the circumference \(2\pi r\) of a great circle.

Answer

a) \(M'(h)=2\pi r\); it is the cylinder base's circumference and the rate at which lateral area changes with height. b) \(S'(r)=8\pi r\); it is the instantaneous rate at which a sphere's surface area changes with radius.
52230512
A cross section of an artificial hill is modeled by \(f(x)=-0.2x^2+2x+1\) for \(0\le x\le 5\), where both coordinates are measured in meters. A straight slide will connect smoothly to the hill at \(x=2\). Find an equation of the line that models the slide.

Hints

- A smooth connection requires the two graphs to share a point and a slope. - Find the point on the hill at \(x=2\). - Use the derivative to find the hill's slope at that point. - Write the line using the point and slope.

Solution

1. Find the connection point: \(f(2)=-0.2(2)^2+2(2)+1=4.2\), so the point is \((2,4.2)\). 2. Differentiate: \(f'(x)=-0.4x+2\). 3. Find the slope at the connection point: \(f'(2)=-0.4(2)+2=1.2\). 4. A smooth connection requires the line to have the same point and slope. Using point-slope form, \(y-4.2=1.2(x-2)\). 5. Simplify: \(y=1.2x+1.8\).

Answer

\(y=1.2x+1.8\)
52656912
The function \(h\) gives the height, in meters, of a giant sequoia \(t\) years after it was planted. Write each statement using function notation with \(h\) and \(h'\). a) When the tree was planted, it was \(1.2\,\text{m}\) tall. b) After exactly \(10\) years, its instantaneous growth rate was \(0.8\,\text{m/year}\). c) From the end of year \(5\) through the end of year \(10\), the tree grew a total of \(3.5\,\text{m}\). d) The tree stopped growing after \(150\) years. e) Its average growth rate during the first \(20\) years was \(0.6\,\text{m/year}\).

Hints

- Decide whether each statement describes a value at one time or a change over an interval. - Recall the difference between an instantaneous rate of change and an average rate of change. - Which notation represents the instantaneous rate of change? - Pay attention to whether the statement gives a height, a total change, or a rate.

Solution

1. The height at the planting time is represented by \(h(0)\), so \(h(0)=1.2\). 2. An instantaneous growth rate is represented by the derivative, so \(h'(10)=0.8\). 3. The total growth from year \(5\) to year \(10\) is the difference in heights, so \(h(10)-h(5)=3.5\). 4. Stopping growth after year \(150\) means the instantaneous growth rate remains zero, so \(h'(t)=0\) for all \(t\geq150\). 5. The average growth rate over \([0,20]\) is the difference quotient, so \(\frac{h(20)-h(0)}{20}=0.6\).

Answer

a) \(h(0)=1.2\) b) \(h'(10)=0.8\) c) \(h(10)-h(5)=3.5\) d) \(h'(t)=0\) for all \(t\geq150\) e) \(\frac{h(20)-h(0)}{20}=0.6\)
52900012
For a circle, the derivative of area \(A(r)=\pi r^2\) equals the circumference \(C(r)=2\pi r\). Investigate analogous relationships for a square and a cube. a) Differentiate the area \(A(x)=x^2\) of a square and compare it with the perimeter \(P(x)=4x\). b) Differentiate the volume \(V(x)=x^3\) of a cube and compare it with the surface area \(S(x)=6x^2\). c) What constant ratio between the derivative and the boundary measure appears in both cases?

Hints

- Differentiate using the power rule. - Compare each derivative directly with the corresponding perimeter or surface-area formula. - Find the constant factor that multiplies the boundary measure to produce the derivative. - Check whether the factor is the same in both cases.

Solution

1. For the square, \(A'(x)=2x\). Since \(P(x)=4x\), \(A'(x)=\frac{1}{2}P(x)\). 2. For the cube, \(V'(x)=3x^2\). Since \(S(x)=6x^2\), \(V'(x)=\frac{1}{2}S(x)\). 3. In both cases, the ratio of the derivative to the boundary measure is \(\frac{1}{2}\).

Answer

a) \(A'(x)=2x=\frac{1}{2}P(x)\) b) \(V'(x)=3x^2=\frac{1}{2}S(x)\) c) The ratio is \(\frac{1}{2}\), or \(1:2\).
52918812
The total number of views of a viral video is modeled by \(V(t)\). An analyst says, “The growth rate of views reached its absolute maximum today.” Assume \(t_w\) is an interior time, the growth rate increases before \(t_w\), and it decreases after \(t_w\). 1. What type of point is \((t_w, V(t_w))\) on the graph of \(V\)? 2. If \(V\) is twice differentiable, what necessary condition holds at \(t_w\)? 3. Describe the concavity before and after \(t_w\).

Hints

- Which derivative represents growth rate? - What must the derivative of a differentiable function equal at an interior maximum? - How does an increasing or decreasing slope relate to concavity? - Picture a graph that first becomes steeper and then begins to flatten.

Solution

1. The growth rate is \(V'\). Since \(V'\) changes from increasing to decreasing at \(t_w\), it has a local maximum there. Thus, the graph of \(V\) changes concavity at \(t_w\), making \((t_w, V(t_w))\) an inflection point. 2. At an interior extremum of \(V'\), Fermat's theorem gives \(V''(t_w)=0\). 3. Before \(t_w\), the growth rate increases, so \(V''(t)>0\) and the graph is concave up. After \(t_w\), the growth rate decreases, so \(V''(t)<0\) and the graph is concave down.

Answer

1. An inflection point 2. \(V''(t_w)=0\) 3. Concave up before \(t_w\); concave down after \(t_w\)
52228712
The functions \(A(x)=x^2\) and \(V(x)=x^3\) model geometric quantities, where \(x\) is a side length or edge length. 1. Find each derivative with respect to \(x\). 2. Explain the geometric meaning of each function and its derivative.

Hints

- What geometric quantities are calculated by \(x^2\) and \(x^3\)? - Use the power rule to differentiate each function. - Imagine increasing a square or cube by a very small amount along adjacent sides or faces. - Compare the units of each derivative with the units of its original function.

Solution

1. By the power rule, \(A'(x)=2x\) and \(V'(x)=3x^2\). 2. The function \(A(x)=x^2\) is the area of a square with side length \(x\). The derivative \(A'(x)=2x\) is the instantaneous rate of change of area with respect to side length, in square units per unit of length. For a small increase \(\Delta x\), the first-order area increase is approximately \(2x\Delta x\), represented by two thin strips along adjacent sides. 3. The function \(V(x)=x^3\) is the volume of a cube with edge length \(x\). The derivative \(V'(x)=3x^2\) is the instantaneous rate of change of volume with respect to edge length, in cubic units per unit of length. For a small increase \(\Delta x\), the first-order volume increase is approximately \(3x^2\Delta x\), represented by three thin slabs along adjacent faces.

Answer

1. \(A'(x)=2x\) and \(V'(x)=3x^2\) 2. \(A\) gives the area of a square, and \(A'\) gives its instantaneous area-growth rate with respect to side length. \(V\) gives the volume of a cube, and \(V'\) gives its instantaneous volume-growth rate with respect to edge length.
52229912
A shot put follows the approximate path \(h(x)=-0.1x^2+0.8x+2\), where \(x\) is the horizontal distance from the release point and \(h(x)\) is the height above the ground, both in meters. Find the acute angle at which the path meets the ground. Round to the nearest tenth of a degree.

Hints

- What equation identifies the point where the object reaches the ground? - The derivative at the impact point gives the tangent slope. - Use the magnitude of the slope when finding the acute angle with the horizontal. - Round the angle as requested.

Solution

1. Find where the shot put reaches the ground by solving \(h(x)=0\). The solutions are \(x=-2\) and \(x=10\), so the physically relevant impact point is \(x=10\). 2. Differentiate: \(h'(x)=-0.2x+0.8\). 3. Find the slope at impact: \(h'(10)=-0.2(10)+0.8=-1.2\). 4. The acute angle \(\alpha\) with the horizontal satisfies \(\tan(\alpha)=|-1.2|=1.2\). Thus, \(\alpha=\arctan(1.2)\approx 50.2^\circ\).

Answer

\(50.2^\circ\)
52230012
The cross section of a skate ramp is modeled for \(0\le x\le 4\) by \(f(x)=\frac{1}{8}x^2-x+2\), where \(x\) and \(f(x)\) are measured in meters. The ramp begins on a platform at \(x=0\) and ends at ground level at \(x=4\). a) Find the acute angle that the ramp makes with the horizontal at \(x=0\). b) Show algebraically that the ramp meets the horizontal ground smoothly, without a corner, at \(x=4\).

Hints

- What does the derivative tell you about the graph at a point? - For a smooth transition to the ground, which two quantities must match? - Use the relationship between slope and angle.

Solution

1. Differentiate: \(f'(x)=\frac{1}{4}x-1\). 2. At \(x=0\), the slope is \(f'(0)=-1\). The acute angle \(\alpha\) satisfies \(\tan(\alpha)=|-1|=1\), so \(\alpha=45^\circ\). 3. At \(x=4\), \(f(4)=\frac{1}{8}\cdot 4^2-4+2=0\), so the ramp reaches the ground. 4. Also, \(f'(4)=\frac{1}{4}\cdot 4-1=0\), which matches the slope of the horizontal line \(y=0\). Since both the height and slope agree, the ramp joins the ground smoothly at \((4,0)\).

Answer

a) \(45^\circ\) b) \(f(4)=0\) and \(f'(4)=0\), so the ramp and the ground line \(y=0\) have the same point and slope at \((4,0)\).
52230612
During one phase of flight, a model airplane follows the path \(g(x)=x^3-4x^2+5x\), where \(x\) and \(g(x)\) are measured in meters. At \(x=2\), the airplane continues along a straight path tangent to the original path. Find an equation of the line that models the new path.

Hints

- A tangent continuation has the same value and slope as the curve at the transition point. - Differentiate the given function. - Use point-slope form for the tangent line. - Verify that the line passes through the point on the curve.

Solution

1. Find the transition point: \(g(2)=2^3-4\cdot 2^2+5\cdot 2=2\), so the point is \((2,2)\). 2. Differentiate: \(g'(x)=3x^2-8x+5\). 3. Find the slope at \(x=2\): \(g'(2)=3\cdot 2^2-8\cdot 2+5=1\). 4. Use point-slope form: \(y-2=1(x-2)\). 5. Simplify: \(y=x\).

Answer

\(y=x\)
52234712
A section of a roller coaster track is modeled by \(f(x)=0.01x^3-0.2x^2+1.5x+2\) for \(0\le x\le 10\), where both coordinates are measured in meters. Starting at \(x=10\), the track continues as a straight line with a smooth connection. Find the horizontal position \(x\) where this straight section reaches a height of \(12\,\text{m}\).

Hints

- Find the point where the straight continuation begins. - A smooth connection requires the line to have the same slope as the curve. - Write the tangent line at \(x=10\). - Set the line's output equal to the target height.

Solution

1. Find the transition point: \(f(10)=0.01(10)^3-0.2(10)^2+1.5(10)+2=7\), so the point is \((10,7)\). 2. Differentiate: \(f'(x)=0.03x^2-0.4x+1.5\). 3. Find the slope at the transition: \(f'(10)=0.03(100)-0.4(10)+1.5=0.5\). 4. A smooth straight continuation is the tangent line: \(y-7=0.5(x-10)\), so \(y=0.5x+2\). 5. Set the height equal to \(12\): \(12=0.5x+2\). Solving gives \(x=20\).

Answer

\(x=20\,\text{m}\)
52234812
During the first \(10\) minutes of cooling, the temperature of a chemical solution is modeled by \(T(t)=0.1t^2-4t+80\), where \(t\) is in minutes and \(T(t)\) is in degrees Celsius. After exactly \(10\) minutes, the temperature continues to decrease linearly with a smooth transition. How many seconds after cooling begins does the solution reach \(25^\circ\text{C}\)?

Hints

- A smooth transition means the linear model has the same value and slope at the joining time. - Find the temperature and rate of change at \(t=10\). - Write the linear model for times after \(10\) minutes. - Convert the final time to the requested unit.

Solution

1. Find the temperature at the transition: \(T(10)=0.1(10)^2-4(10)+80=50\). 2. Differentiate: \(T'(t)=0.2t-4\). The rate at \(t=10\) is \(T'(10)=-2\) degrees Celsius per minute. 3. A smooth linear continuation has the same value and rate at \(t=10\): \(L(t)-50=-2(t-10)\), so \(L(t)=-2t+70\). 4. Solve \(25=-2t+70\). This gives \(t=22.5\) minutes. 5. Convert to seconds: \(22.5\cdot 60=1350\) seconds.

Answer

\(1350\,\text{s}\)
52235912
The elevation profile of a mountain-bike trail is modeled for \(0\le x\le 30\) by \(h(x)=-0.002x^3+0.06x^2+10\), where \(x\) and \(h(x)\) are measured in meters. a) Find the trail's grade at \(x=5\), expressed as a percentage. b) Find the angle of inclination at \(x=15\). c) Find an equation of the tangent line to the elevation profile at \(x=25\).

Hints

- The derivative gives the local slope of the trail. - How are slope, percent grade, and angle of inclination related? - A tangent line requires both the point and the slope. - Use point-slope form for the tangent line.

Solution

1. Differentiate: \(h'(x)=-0.006x^2+0.12x\). 2. At \(x=5\), \(h'(5)=-0.006(25)+0.12(5)=0.45\). The percent grade is \(0.45\cdot 100\%=45\%\). 3. At \(x=15\), \(h'(15)=0.45\). Therefore, \(\alpha=\arctan(0.45)\approx 24.23^\circ\). 4. At \(x=25\), the slope is \(h'(25)=-0.75\), and \(h(25)=16.25\). The tangent line is \(y-16.25=-0.75(x-25)\), so \(y=-0.75x+35\).

Answer

a) \(45\%\) b) \(24.23^\circ\), approximately c) \(y=-0.75x+35\)
52238112
The cross-section of a trough is modeled by \(f(x)=0.2x^2-2\), with coordinates measured in feet. A support is attached at \(P(5,3)\) perpendicular to the side of the trough. Its straight extension meets a horizontal cover along the line \(y=7\). Find the coordinates of the intersection point \(Q\).

Hints

- A line perpendicular to the curve at a point is the normal line. - Use the derivative to find the curve's tangent slope. - Tangent and normal slopes are negative reciprocals. - Intersect the normal line with \(y=7\).

Solution

1. Differentiate: \(f'(x)=0.4x\). At \(x=5\), the tangent slope is \(f'(5)=2\). 2. The support follows the normal line, whose slope is \(-\frac{1}{2}\). 3. Through \(P(5,3)\), the normal line is \(y-3=-0.5(x-5)\), or \(y=-0.5x+5.5\). 4. Set \(y=7\): \(7=-0.5x+5.5\), so \(x=-3\). 5. Therefore, \(Q=(-3,7)\).

Answer

\(Q=(-3,7)\)
52239612
During a storm, the water volume in a retention basin is modeled for the first \(12\) minutes by \(W(t) = -0.2t^3 + 3t^2 + 20t\), where \(W(t)\) is measured in cubic meters and \(t\) is the number of minutes since measurements began. a) Explain what \(W''(t)\) means in this context. b) Find \(W''(2)\) and \(W''(8)\). Interpret the signs in terms of how the basin's net inflow rate is changing. c) Explain mathematically why this model should not be extended beyond the stated interval, such as to \(t = 15\), if the volume is expected to keep increasing.

Hints

- If the first derivative is a rate, what does the derivative of that rate represent? - What does the sign of the second derivative tell you about how the first derivative changes? - What must be true of \(W'(t)\) while the water volume is increasing? - Examine the value of the first derivative at a time beyond the model's interval.

Solution

1. The first derivative \(W'(t)\) is the net rate of change of the water volume. Therefore, \(W''(t)\) is the rate at which that net inflow rate is changing, measured in \(\frac{\text{m}^3}{\text{min}^2}\). 2. Differentiate: \(W'(t) = -0.6t^2 + 6t + 20\) and \(W''(t) = -1.2t + 6\). Then \(W''(2)=3.6\,\frac{\text{m}^3}{\text{min}^2}\), so the net inflow rate is increasing at \(t=2\). Also, \(W''(8)=-3.6\,\frac{\text{m}^3}{\text{min}^2}\), so the net inflow rate is decreasing at \(t=8\). 3. Extending the model to \(t=15\) gives \(W'(15)=-0.6 \cdot 15^2 + 6 \cdot 15 + 20 = -25\,\frac{\text{m}^3}{\text{min}}\). This negative rate would mean the water volume is decreasing, so the extended model would no longer describe a continuously filling basin.

Answer

a) \(W''(t)\) is the rate of change of the basin's net inflow rate. b) \(W''(2)=3.6\,\frac{\text{m}^3}{\text{min}^2}\), so the net inflow rate is increasing. \(W''(8)=-3.6\,\frac{\text{m}^3}{\text{min}^2}\), so the net inflow rate is decreasing. c) \(W'(15)=-25\,\frac{\text{m}^3}{\text{min}}\), which would make the modeled volume decrease.
52259312
The function \(h\) gives a sunflower's height, in centimeters, \(t\) days after germination. Assume \(h\) is three times differentiable. For each situation, select every card that must be true. (1) After \(12\) days, the plant is \(80\,\text{cm}\) tall. (2) After \(6\) days, the growth rate has a local maximum. (3) After \(25\) days, the height has a local maximum of \(160\,\text{cm}\). Cards: A: \(h(12)=80\) B: \(h''(6)=0\) C: \(h(25)=160\) D: \(h'(25)=0\) E: \(h''(25)<0\) F: \(h'(6)>0\) G: \(h'''(6)<0\)

Hints

- Distinguish conditions that must be true from conditions that are only sufficient. - The growth rate is the first derivative. - At an interior local extremum of a differentiable function, its derivative is \(0\). - A negative second derivative can confirm a local maximum, but it is not required.

Solution

1. Situation (1) directly states the function value \(h(12)=80\), so card A must be true. 2. The growth rate is \(h'\). If \(h'\) has a local maximum at \(t=6\), Fermat's theorem applied to \(h'\) gives \(h''(6)=0\). The value \(h'(6)\) need not be positive, and \(h'''(6)<0\) is a sufficient but not necessary test for the maximum. Thus only card B must be true. 3. A local maximum height of \(160\,\text{cm}\) at \(t=25\) gives \(h(25)=160\). Since \(h\) is differentiable, Fermat's theorem gives \(h'(25)=0\). The condition \(h''(25)<0\) is sufficient but not necessary for a local maximum. Thus cards C and D must be true.

Answer

(1) A (2) B (3) C, D
52259412
A model gives a drone's altitude \(H\), in meters, as a function of flight time \(t\), in seconds. Assume \(H\) is three times differentiable. For each situation, select every card that must be true. (1) At \(t=10\), the drone is descending at its greatest local rate. (2) At \(t=30\), the drone reaches a local maximum altitude of \(120\,\text{m}\). Cards: A: \(H(30)=120\) B: \(H'(30)=0\) C: \(H''(30)<0\) D: \(H''(10)=0\) E: \(H'(10)<0\) F: \(H'''(10)>0\)

Hints

- Distinguish conditions that must be true from conditions that are only sufficient. - The first derivative represents vertical velocity. - “Descending” determines the sign of the velocity. - At an interior local extremum of a differentiable function, its derivative is \(0\).

Solution

1. Descending at the greatest local rate means that the vertical velocity \(H'\) has a local minimum at \(t=10\) and is negative there. Fermat's theorem applied to \(H'\) gives \(H''(10)=0\), and descending gives \(H'(10)<0\). The condition \(H'''(10)>0\) is sufficient but not necessary for the local minimum. Thus cards D and E must be true. 2. A local maximum altitude of \(120\,\text{m}\) at \(t=30\) gives \(H(30)=120\). Since \(H\) is differentiable, Fermat's theorem gives \(H'(30)=0\). The condition \(H''(30)<0\) is sufficient but not necessary for a local maximum. Thus cards A and B must be true.

Answer

(1) D, E (2) A, B
52737712
The concentration of a dye in a chemical solution is modeled by \(c(t)=\frac{10t}{(t+4)^2}\), where \(c(t)\) is measured in grams per liter and \(t\ge 0\) is the time in hours after the solution is prepared. a) Explain what \(c'(t)\) means in this context and give its units. b) Find the time when the dye concentration is greatest. State when the concentration is increasing and when it is decreasing. c) Find and interpret the instantaneous rates of change at \(t=0\), \(t=4\), and \(t=8\). d) Find the average rate of change of the concentration over \([0,8]\). Compare it with the instantaneous rate of change at \(t=4\), and explain the difference.

Hints

- What does the slope of a function represent in a real-world context? - How can a derivative be used to locate a maximum? - What is the difference between a slope at one instant and an average slope over an interval? - What signs should the derivative have when a function is increasing or decreasing?

Solution

1. The derivative \(c'(t)\) gives the instantaneous rate of change of dye concentration. Its units are \(\frac{\text{g}}{\text{L}\cdot\text{h}}\). 2. Differentiate using the quotient rule: \(c'(t)=\frac{10(t+4)^2-20t(t+4)}{(t+4)^4}=\frac{40-10t}{(t+4)^3}\). 3. Set the derivative equal to zero: \(40-10t=0\), so \(t=4\). Because \(c'(t)>0\) for \(0\le t<4\) and \(c'(t)<0\) for \(t>4\), the concentration is greatest at \(t=4\,\text{h}\). It increases on \([0,4)\) and decreases on \((4,\infty)\). 4. Evaluate the derivative: \(c'(0)=\frac{5}{8}=0.625\), \(c'(4)=0\), and \(c'(8)=-\frac{5}{216}\approx -0.0231\), all in \(\frac{\text{g}}{\text{L}\cdot\text{h}}\). Thus the concentration is initially increasing, is momentarily unchanged at its maximum, and is decreasing at \(t=8\). 5. The average rate of change is \(\frac{c(8)-c(0)}{8-0}=\frac{5}{72}\approx 0.0694\,\frac{\text{g}}{\text{L}\cdot\text{h}}\). 6. This average is positive because the concentration at \(t=8\) is greater than at \(t=0\), even though the instantaneous rate at \(t=4\) is zero because the concentration has a maximum there.

Answer

a) \(c'(t)\) is the instantaneous rate of change of concentration, measured in \(\frac{\text{g}}{\text{L}\cdot\text{h}}\). b) The maximum occurs at \(t=4\,\text{h}\). The concentration increases on \([0,4)\) and decreases on \((4,\infty)\). c) \(c'(0)=0.625\,\frac{\text{g}}{\text{L}\cdot\text{h}}\), \(c'(4)=0\,\frac{\text{g}}{\text{L}\cdot\text{h}}\), and \(c'(8)=-\frac{5}{216}\,\frac{\text{g}}{\text{L}\cdot\text{h}}\approx -0.0231\,\frac{\text{g}}{\text{L}\cdot\text{h}}\). d) The average rate is \(\frac{5}{72}\,\frac{\text{g}}{\text{L}\cdot\text{h}}\approx 0.0694\,\frac{\text{g}}{\text{L}\cdot\text{h}}\). It is positive, while \(c'(4)=0\), because \(t=4\) is the instant when the concentration reaches its maximum.
52899912
A circular cylinder has fixed height \(h=10\,\text{cm}\). Its volume as a function of radius is \(V(r)=10\pi r^2\). a) Find \(V'(r)\). b) The lateral surface area is \(M(r)=2\pi rh\). Compare \(V'(r)\) with \(M(r)\) when \(h=10\,\text{cm}\). c) Explain the relationship by considering a very small increase in radius.

Hints

- Differentiate a constant multiple of \(r^2\). - Substitute the fixed height into the lateral-area formula and compare expressions. - Expand the exact volume difference for radius \(r+\Delta r\) and identify its first-order term.

Solution

1. Differentiate: \(V'(r)=20\pi r\). 2. With \(h=10\), the lateral area is \(M(r)=2\pi r(10)=20\pi r\). Thus \(V'(r)=M(r)\). 3. If the radius increases by a small amount \(\Delta r\), then \(\Delta V=10\pi[(r+\Delta r)^2-r^2]=20\pi r\Delta r+10\pi(\Delta r)^2\). The first-order change is \(20\pi r\Delta r=M(r)\Delta r\); the quadratic term becomes negligible relative to \(\Delta r\) in the derivative limit.

Answer

a) \(V'(r)=20\pi r\) b) \(V'(r)=M(r)\) c) For a very small radial increase, the first-order volume change is the lateral surface area times the radial change.
52918712
A market report says, “The decline in stock prices is slowing.” Let a twice differentiable function \(f(t)\) represent a stock price over time. 1. What does “decline” indicate about the sign of \(f'(t)\)? 2. What does “slowing” indicate about the sign of \(f''(t)\)? 3. Describe the graph's concavity during the slowing decline, and name the type of point that marks a transition from an accelerating decline to a slowing decline.

Hints

- Is the function increasing or decreasing? - If a decline slows, does the slope become more negative or less negative? - Relate the sign of the second derivative to concavity. - What type of point separates intervals of different concavity?

Solution

1. A decline means the stock price is decreasing, so \(f'(t)<0\). 2. A slowing decline means the negative slope is becoming less negative. Therefore, the slope is increasing and \(f''(t)>0\). 3. Since \(f''(t)>0\), the graph is concave up during the slowing decline. A transition from \(f''<0\) to \(f''>0\) is an inflection point and corresponds to the most negative slope.

Answer

1. \(f'(t)<0\) 2. \(f''(t)>0\) 3. The graph is concave up. The transition point is an inflection point.
53239912
In an experiment, a small laser moves along a rail whose profile is modeled by \(f(x)=-0.25x^2+4\), for \(0\le x\le4\). The laser always points to the right along the tangent line to the rail. A light sensor is located at \(S(4,1)\), as shown. Find the position \(x_0\) of the laser so that its beam hits the sensor.
Figure for problem 532399

Hints

- Write the tangent-line equation at a general input \(x_0\). - Use the derivative for the tangent slope. - Substitute the sensor coordinates into the tangent-line equation. - Solve the resulting quadratic equation. - Check each solution against the stated domain.

Solution

1. Differentiate: \(f'(x)=-0.5x\). 2. The tangent line at \(x=x_0\) is \(t(x)=f'(x_0)(x-x_0)+f(x_0)\). Substitution and simplification give \(t(x)=-0.5x_0x+0.25x_0^2+4\). 3. Since the line must pass through \(S(4,1)\), set \(t(4)=1\): \(1=-2x_0+0.25x_0^2+4\). 4. Rearranging gives \(x_0^2-8x_0+12=0\), so \((x_0-2)(x_0-6)=0\). Thus, \(x_0=2\) or \(x_0=6\). 5. Only \(x_0=2\) lies in the rail's domain \([0,4]\).

Answer

The laser must be at \(x_0=2\).
53253312
A walking path in a city park follows the parabola \(f(x)=-0.5x^2+2x+1.5\), where each coordinate unit represents \(10\,\text{m}\). A straight connector path will branch off perpendicular to the walking path at \(A(3,3)\). It crosses a creek modeled by \(g(x)=-2x+6\), where a small bridge will be built. Find the equation of the connector path and the coordinates of the bridge location \(B\).
Figure for problem 532533

Hints

- Perpendicular lines have slopes that are negative reciprocals. - Use the derivative to find the walking path's tangent slope at \(A\). - Write the connector line through \(A\). - Set the connector and creek equations equal to find their intersection.

Solution

1. Differentiate: \(f'(x)=-x+2\). At \(x=3\), the tangent slope is \(f'(3)=-1\). 2. The perpendicular connector has slope \(1\). 3. Through \(A(3,3)\), its equation is \(y-3=x-3\), or \(y=x\). 4. Intersect this line with the creek: \(x=-2x+6\). Thus, \(3x=6\), so \(x=2\) and \(y=2\). 5. Therefore, the bridge location is \(B=(2,2)\).

Answer

The connector path is \(y=x\), and the bridge is at \(B=(2,2)\).
53370212
A ski-jump profile is modeled by \(f(x)=-0.1x^3+0.6x^2\) for \(0\le x\le6\), with distances measured in meters. A skier leaves the ramp at \(x=2\), and the initial path is approximated by the tangent line to the graph. A vertical safety wall is located at \(x=6\). At what height \(h\) above the ground does this tangent-line model predict that the skier reaches the wall?
Figure for problem 533702

Hints

- First find the exact takeoff point on the ramp. - Use the derivative to find the ramp's slope at takeoff. - Model the initial path with the tangent line. - Substitute the wall's x-coordinate into the tangent-line equation.

Solution

1. Find the takeoff point: \(f(2)=-0.1\cdot2^3+0.6\cdot2^2=1.6\), so the point is \((2,1.6)\). 2. Differentiate: \(f'(x)=-0.3x^2+1.2x\). 3. Find the slope at takeoff: \(f'(2)=-0.3\cdot2^2+1.2\cdot2=1.2\). 4. The tangent line is \(y-1.6=1.2(x-2)\), or \(y=1.2x-0.8\). 5. At the wall, \(y=1.2\cdot6-0.8=6.4\). Therefore, \(h=6.4\,\text{m}\).

Answer

\(h=6.4\,\text{m}\)
53422112
The graph shows the temperature \(T\), in degrees Celsius, in a sunroom over \(10\) hours. A sensor also records the instantaneous rate of change of temperature in degrees Celsius per hour. Which graph, 1, 2, 3, or 4, represents this rate of change? Justify your choice using relationships between a function and its derivative, including extrema and monotonicity.
Figure for problem 534221

Hints

- The rate of change is zero at local maxima and minima of the temperature graph. - Determine when the temperature is increasing or decreasing. - Match the candidate zeros and signs to the temperature graph. - Use the steepest parts of the temperature graph as an additional check.

Solution

1. The temperature graph has a local maximum at \(t=3\) and a local minimum at \(t=7\). 2. The rate-of-change graph must have zeros at those times because the temperature graph has horizontal tangent lines there. 3. Graphs 1 and 2 both have zeros at \(t=3\) and \(t=7\). Graph 3 has zeros at \(t=1\) and \(t=5\), and graph 4 has no zeros on the displayed interval. Thus, only graphs 1 and 2 remain possible. 4. The temperature decreases between \(t=3\) and \(t=7\), so its rate of change must be negative there. Graph 1 is negative on this interval, while graph 2 is positive. Therefore, graph 1 represents the rate of change.

Answer

Graph 1 represents the instantaneous rate of change of temperature.
53436212
The graph of \(N\) models the number of users of a new streaming service during its first \(10\) months. The value \(N(t)\) is measured in millions of users, and \(t\) is measured in months. a) At what time is the number of users increasing most rapidly? b) Interpret line \(g\) in context. c) Describe the graph of \(N^{\prime}\), and explain what it shows about the rate of user growth.
Figure for problem 534362

Hints

- The greatest rate of increase occurs where the graph is steepest. - Interpret a horizontal asymptote as a limiting value in context. - The derivative records the slope of the original graph. - Compare slopes before and after the inflection point.

Solution

1. The user count increases most rapidly where the graph of \(N\) has its greatest slope, at the inflection point of the S-shaped curve. This occurs at \(t=5\) months. 2. Line \(g\) is the horizontal asymptote \(N=12\). In context, it represents a long-term saturation level of \(12\) million users. 3. The derivative \(N^{\prime}\) gives the instantaneous growth rate in millions of users per month. Its graph is bell-shaped, stays positive, and has its maximum at \(t=5\). Thus, the growth rate increases until month \(5\) and then decreases as the user count approaches saturation.

Answer

a) At \(t=5\) months b) Line \(g\) represents a long-term saturation level of \(12\) million users. c) The graph of \(N^{\prime}\) is a positive bell-shaped curve with a maximum at \(t=5\). It represents the instantaneous user-growth rate.
53454112
The concentration of a medication in a patient's blood is modeled by \(k(t)=\frac{80}{(t-10)^2+12}\), where \(t\ge0\) is time in hours after the dose and \(k(t)\) is measured in \(\text{mg/L}\). a) Find the time and value of the maximum concentration. b) For \(t>10\), the graph has an inflection point at \(t=12\). Find the concentration then and interpret the inflection point in context.
Figure for problem 534541

Hints

- A fraction with a fixed positive numerator is largest when its denominator is smallest. - Substitute \(t=12\) into the model. - At an inflection point, the rate of change reaches a local extremum.

Solution

1. The numerator is constant, so \(k(t)\) is greatest when the denominator is smallest. The denominator \((t-10)^2+12\) is minimized at \(t=10\). 2. The maximum concentration is \(k(10)=\frac{80}{12}=\frac{20}{3}\approx6.67\,\text{mg/L}\). 3. At the inflection point, \(k(12)=\frac{80}{(12-10)^2+12}=\frac{80}{16}=5\,\text{mg/L}\). 4. On the decreasing side of the curve, the inflection point is where \(k'(t)\) is most negative. Thus, at \(t=12\), the medication concentration is decreasing at its fastest rate.

Answer

a) Maximum at \(t=10\) hours: \(\frac{20}{3}\approx6.67\,\text{mg/L}\) b) \(k(12)=5\,\text{mg/L}\). At that time, the concentration is decreasing at its fastest rate.
53385012
A surveillance drone follows the path \(g(x)=\sin(0.5x)+2\). It can release a laser pulse that travels along the tangent line to the path. A ground sensor is at \(P(10,0.142)\). At which candidate input, \(x_1=\frac{3\pi}{2}\), \(x_2=2\pi\), or \(x_3=\frac{5\pi}{2}\), should the pulse be released to hit the sensor? Round all intermediate values to three decimal places.
Figure for problem 533850

Hints

- A tangential pulse follows the tangent line at the release point. - Use the derivative to find the tangent slope at each candidate input. - Write each tangent line using its release point and slope. - Evaluate each tangent line at the sensor's x-coordinate.

Solution

1. Differentiate: \(g'(x)=0.5\cos(0.5x)\). 2. At \(x_1=\frac{3\pi}{2}\approx4.712\), \(g(x_1)\approx2.707\) and \(g'(x_1)\approx-0.354\). The tangent gives \(y\approx-0.354(10-4.712)+2.707\approx0.835\), which does not match the sensor height. 3. At \(x_2=2\pi\approx6.283\), \(g(x_2)\approx2.000\) and \(g'(x_2)\approx-0.500\). The tangent gives \(y\approx-0.500(10-6.283)+2.000\approx0.142\), which matches the sensor height. 4. At \(x_3=\frac{5\pi}{2}\approx7.854\), \(g(x_3)\approx1.293\) and \(g'(x_3)\approx-0.354\). The tangent gives \(y\approx-0.354(10-7.854)+1.293\approx0.533\), which does not match the sensor height. 5. Therefore, the pulse should be released at \(x_2=2\pi\).

Answer

\(x_2=2\pi\)
53489812
A snowboarder follows an approximately parabolic path after leaving a ramp at \((0,0)\). At a horizontal distance of \(10\,\text{m}\), the snowboarder is \(5\,\text{m}\) above the takeoff level. The highest point occurs \(6\,\text{m}\) horizontally from the takeoff point. a) Translate “the takeoff point is the origin” and “the highest point occurs at a horizontal distance of \(6\,\text{m}\)” into conditions on \(g\) and \(g'\). b) Find the quadratic function \(g\) that models the path. c) Find the maximum height above the takeoff level. d) The snowboarder lands on level ground at a horizontal distance of \(12\,\text{m}\). Find the magnitude of the impact angle with the ground.
Figure for problem 534898

Hints

- A point on the path gives a condition on the function value. - At the highest point, the tangent is horizontal. - Start with a general quadratic and solve for its coefficients. - The tangent slope and the angle with the horizontal are related by tangent.

Solution

1. The takeoff condition is \(g(0)=0\). A horizontal tangent at the highest point gives \(g'(6)=0\). The remaining given point gives \(g(10)=5\). 2. Let \(g(x)=ax^2+bx+c\). From \(g(0)=0\), \(c=0\). Since \(g'(x)=2ax+b\), the condition \(g'(6)=0\) gives \(12a+b=0\), so \(b=-12a\). Using \(g(10)=5\), \(100a+10b=5\). Substitution gives \(-20a=5\), so \(a=-\frac{1}{4}\) and \(b=3\). Therefore, \(g(x)=-\frac{1}{4}x^2+3x\). 3. The maximum occurs at \(x=6\): \(g(6)=-\frac{1}{4}(36)+18=9\). The maximum height is \(9\,\text{m}\). 4. The slope at landing is \(g'(12)=-\frac{1}{2}(12)+3=-3\). If \(\theta\) is the magnitude of the angle with the horizontal ground, then \(\tan(\theta)=3\). Thus \(\theta=\arctan(3)\approx71.6^\circ\).

Answer

a) \(g(0)=0\) and \(g'(6)=0\). b) \(g(x)=-\frac{1}{4}x^2+3x\). c) \(9\,\text{m}\). d) \(\arctan(3)\approx71.6^\circ\).

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