Average Value of a Function - Calculus 1

  1. Let f(x)=xf(x) = \sqrt{x}. Let A1A_1 be the average value of ff on the interval [0,1][0, 1] and let A2A_2 be the average value of ff on the interval [1,4][1, 4]. Which of the following statements correctly describes the relationship between A1A_1 and A2A_2?
    Answer: A1<A2A_1 < A_2 The function f(x)=xf(x) = \sqrt{x} is a strictly increasing function for x>0x > 0. The average value of a strictly increasing function over an interval [c,d][c,d] will always be greater than its average value over a preceding interval [a,b][a,b] (where b≤cb \le c). Therefore, A1<A2A_1 < A_2. By computation: A1=11∫01x1/2 dx=[23x3/2]01=23A_1 = \frac{1}{1} \int_0^1 x^{1/2} \,dx = [\frac{2}{3}x^{3/2}]_0^1 = \frac{2}{3}. A2=13∫14x1/2 dx=13[23x3/2]14=29(8−1)=149A_2 = \frac{1}{3} \int_1^4 x^{1/2} \,dx = \frac{1}{3}[\frac{2}{3}x^{3/2}]_1^4 = \frac{2}{9}(8-1) = \frac{14}{9}. Since 23=69\frac{2}{3} = \frac{6}{9}, we have 69<149\frac{6}{9} < \frac{14}{9}, confirming that A1<A2A_1 < A_2.
  2. The average value of the function f(x)=kxf(x) = k\sqrt{x} on the interval [1,4][1, 4] is 7. What is the value of the constant kk?
    Answer: 92\frac{9}{2} The average value is given by favg=14−1∫14kx dxf_{avg} = \frac{1}{4-1} \int_1^4 k\sqrt{x} \,dx. We set this equal to 7. 7=k3∫14x1/2 dx=k3[23x3/2]14=2k9[43/2−13/2]=2k9[8−1]=14k97 = \frac{k}{3} \int_1^4 x^{1/2} \,dx = \frac{k}{3} [\frac{2}{3}x^{3/2}]_1^4 = \frac{2k}{9}[4^{3/2} - 1^{3/2}] = \frac{2k}{9}[8-1] = \frac{14k}{9}. Solving for kk: 7=14k9  ⟹  63=14k  ⟹  k=6314=927 = \frac{14k}{9} \implies 63 = 14k \implies k = \frac{63}{14} = \frac{9}{2}. Distractor A results from forgetting the 1b−a\frac{1}{b-a} factor. Distractor B results from using an incorrect interval length of 4. Distractor C results from an error in evaluating the exponent.
  3. For what value of k>1k > 1 is the average value of f(x)=exf(x) = e^x on the interval [0,ln⁡k][0, \ln k] equal to k−1k-1?
    Answer: ee The average value of f(x)=exf(x)=e^x on [0,ln⁡k][0, \ln k] is 1ln⁡k−0∫0ln⁡kex dx=1ln⁡k[ex]0ln⁡k=eln⁡k−e0ln⁡k=k−1ln⁡k\frac{1}{\ln k - 0} \int_0^{\ln k} e^x \,dx = \frac{1}{\ln k} [e^x]_0^{\ln k} = \frac{e^{\ln k} - e^0}{\ln k} = \frac{k-1}{\ln k}. We are given that this average value is equal to k−1k-1. So, k−1ln⁡k=k−1\frac{k-1}{\ln k} = k-1. Since k>1k>1, k−1≠0k-1 \neq 0, and we can divide both sides by k−1k-1 to get 1ln⁡k=1\frac{1}{\ln k} = 1, which implies ln⁡k=1\ln k = 1. Therefore, k=ek=e.
  4. What is the average value of f(x)=ln⁡(x)f(x) = \ln(x) on the interval [1,e][1, e]?
    Answer: 1e−1\frac{1}{e-1} The average value is 1e−1∫1eln⁡(x) dx\frac{1}{e-1} \int_1^e \ln(x) \,dx. The antiderivative of ln⁡(x)\ln(x) is xln⁡(x)−xx \ln(x) - x. Evaluating the definite integral: [xln⁡(x)−x]1e=(eln⁡e−e)−(1ln⁡1−1)=(e−e)−(0−1)=1[x \ln(x) - x]_1^e = (e \ln e - e) - (1 \ln 1 - 1) = (e - e) - (0 - 1) = 1. The average value is 1e−1⋅1=1e−1\frac{1}{e-1} \cdot 1 = \frac{1}{e-1}. Distractor A is the value of the integral. Distractor B incorrectly multiplies the integral by the interval length. Distractor C results from an error in finding the antiderivative.
  5. The temperature in a room over a 12-hour period is modeled by T(t)=68+4sin⁡(πt12)T(t) = 68 + 4\sin(\frac{\pi t}{12}), where tt is in hours from the start time. What is the average temperature, in degrees, during the first 6 hours (from t=0t=0 to t=6t=6)?
    Answer: 68+8π68 + \frac{8}{\pi} When you encounter a problem asking for average temperature over a time interval, you're dealing with the average value of a function. The average value of a continuous function f(t)f(t) over interval [a,b][a,b] is given by 1b−a∫abf(t) dt\frac{1}{b-a}\int_a^b f(t)\,dt. For this problem, you need the average of T(t)=68+4sin⁡(πt12)T(t) = 68 + 4\sin(\frac{\pi t}{12}) from t=0t=0 to t=6t=6: Average=16−0∫06[68+4sin⁡(πt12)]dt\text{Average} = \frac{1}{6-0}\int_0^6 \left[68 + 4\sin(\frac{\pi t}{12})\right]dt =16[∫0668 dt+∫064sin⁡(πt12) dt]= \frac{1}{6}\left[\int_0^6 68\,dt + \int_0^6 4\sin(\frac{\pi t}{12})\,dt\right] The first integral equals 68⋅6=40868 \cdot 6 = 408. For the second integral, use substitution u=πt12u = \frac{\pi t}{12}, so du=π12dtdu = \frac{\pi}{12}dt: ∫064sin⁡(πt12) dt=4⋅12π[−cos⁡(πt12)]06=48π[−cos⁡(π2)+cos⁡(0)]=48π[0+1]=48π\int_0^6 4\sin(\frac{\pi t}{12})\,dt = 4 \cdot \frac{12}{\pi}\left[-\cos(\frac{\pi t}{12})\right]_0^6 = \frac{48}{\pi}[-\cos(\frac{\pi}{2}) + \cos(0)] = \frac{48}{\pi}[0 + 1] = \frac{48}{\pi} Therefore: Average=16[408+48π]=68+8π\text{Average} = \frac{1}{6}\left[408 + \frac{48}{\pi}\right] = 68 + \frac{8}{\pi} Answer C is correct. Answer A (70) ignores the sine component entirely. Answer B has the wrong sign—it subtracts 8π\frac{8}{\pi} instead of adding it. Answer D uses 2π3\frac{2\pi}{3} instead of 8π\frac{8}{\pi}, likely from incorrectly handling the integral bounds or substitution. Study tip: When finding average values involving trigonometric functions, always check your integration limits carefully after substitution, and remember that the average value formula requires dividing by the interval length.
  6. The average value of the derivative of a differentiable function, f′(x)f'(x), on an interval [a,b][a, b] represents which quantity?
    Answer: The average rate of change of the function f(x)f(x) on [a,b][a, b]. The average value of f′(x)f'(x) on [a,b][a, b] is given by the expression 1b−a∫abf′(x) dx\frac{1}{b-a} \int_a^b f'(x) \,dx. By the Fundamental Theorem of Calculus, ∫abf′(x) dx=f(b)−f(a)\int_a^b f'(x) \,dx = f(b) - f(a). Therefore, the expression becomes f(b)−f(a)b−a\frac{f(b)-f(a)}{b-a}, which is the definition of the average rate of change of f(x)f(x) on the interval [a,b][a, b].
  7. The average value of the function f(x)=kx2+1f(x) = kx^2 + 1 on the interval [0,3][0, 3] is 1313. What is the value of the constant kk?
    Answer: 44 The average value of a function f(x)f(x) on an interval [a,b][a, b] is given by the formula favg=1b−a∫abf(x) dxf_{avg} = \frac{1}{b-a} \int_a^b f(x) \, dx. We are given that the average value is 1313 on the interval [0,3][0, 3]. 1. Set up the equation based on the formula: 13=13−0∫03(kx2+1) dx13 = \frac{1}{3-0} \int_0^3 (kx^2 + 1) \, dx 2. Calculate the definite integral: ∫03(kx2+1) dx=[k3x3+x]03=(k3(3)3+3)−(0)=9k+3\int_0^3 (kx^2 + 1) \, dx = \left[ \frac{k}{3}x^3 + x \right]_0^3 = \left( \frac{k}{3}(3)^3 + 3 \right) - (0) = 9k + 3 3. Substitute the result back into the average value equation and solve for kk: 13=13(9k+3)13 = \frac{1}{3}(9k + 3) 13=3k+113 = 3k + 1 12=3k12 = 3k k=4k = 4 Distractor B arises from forgetting to divide by the interval length (b−a)=3(b-a)=3, leading to the incorrect equation 13=9k+313 = 9k+3. Distractor C results from ignoring the '+1+1' term in the function f(x)f(x). Distractor D is the result of a simple arithmetic error when solving 13=3k+113 = 3k+1.
  8. Let favgf_{avg} be the average value of f(x)=sec⁡2(x)f(x)=\sec^2(x) on the interval [0,π/4][0, \pi/4]. Let gavgg_{avg} be the average value of g(x)=cos⁡(x)g(x)=\cos(x) on the interval [0,π/2][0, \pi/2]. What is the value of the product favg⋅gavgf_{avg} \cdot g_{avg}?
    Answer: 8/π28/\pi^2 This question tests your understanding of the average value formula for continuous functions. When finding the average value of a function over an interval, you use the formula: average value = 1b−a∫abf(x)dx\frac{1}{b-a}\int_a^b f(x)dx. For favgf_{avg}, you need the average value of sec⁡2(x)\sec^2(x) on [0,π/4][0, \pi/4]. Since ∫sec⁡2(x)dx=tan⁡(x)\int \sec^2(x)dx = \tan(x), you get: favg=1π/4−0∫0π/4sec⁡2(x)dx=4π[tan⁡(x)]0π/4=4π(1−0)=4πf_{avg} = \frac{1}{\pi/4 - 0}\int_0^{\pi/4} \sec^2(x)dx = \frac{4}{\pi}[\tan(x)]_0^{\pi/4} = \frac{4}{\pi}(1-0) = \frac{4}{\pi} For gavgg_{avg}, you need the average value of cos⁡(x)\cos(x) on [0,π/2][0, \pi/2]. Since ∫cos⁡(x)dx=sin⁡(x)\int \cos(x)dx = \sin(x), you get: gavg=1π/2−0∫0π/2cos⁡(x)dx=2π[sin⁡(x)]0π/2=2π(1−0)=2πg_{avg} = \frac{1}{\pi/2 - 0}\int_0^{\pi/2} \cos(x)dx = \frac{2}{\pi}[\sin(x)]_0^{\pi/2} = \frac{2}{\pi}(1-0) = \frac{2}{\pi} Therefore: favg⋅gavg=4π⋅2π=8π2f_{avg} \cdot g_{avg} = \frac{4}{\pi} \cdot \frac{2}{\pi} = \frac{8}{\pi^2} Answer A) 11 likely comes from forgetting to apply the average value formula and just evaluating the functions at specific points. Answer B) 4π2\frac{4}{\pi^2} results from incorrectly calculating one of the integrals, possibly getting tan⁡(π/4)=12\tan(\pi/4) = \frac{1}{2} instead of 11. Answer D) 2π\frac{2}{\pi} is just one of the individual averages, suggesting you forgot to multiply them together. Remember: always set up the average value formula completely before substituting limits, and double-check your antiderivatives and arithmetic when multiplying fractions.
  9. Let f(x)=3x2−2xf(x) = 3x^2 - 2x. The average value of ff on the interval [0,k][0, k] is equal to the value of the function at x=kx=k, where k>0k>0. What is the value of kk?
    Answer: 1/21/2 This problem combines the average value of a function with the Mean Value Theorem concept. When you see "average value equals function value," you're looking for a point where the function's actual value matches its average over an interval. The average value of f(x)=3x2−2xf(x) = 3x^2 - 2x on [0,k][0, k] is: Average=1k−0∫0k(3x2−2x) dx=1k[x3−x2]0k=k3−k2k=k2−k\text{Average} = \frac{1}{k-0} \int_0^k (3x^2 - 2x) \, dx = \frac{1}{k} \left[ x^3 - x^2 \right]_0^k = \frac{k^3 - k^2}{k} = k^2 - k We need this average to equal f(k)=3k2−2kf(k) = 3k^2 - 2k: k2−k=3k2−2kk^2 - k = 3k^2 - 2k k2−k−3k2+2k=0k^2 - k - 3k^2 + 2k = 0 −2k2+k=0-2k^2 + k = 0 k(−2k+1)=0k(-2k + 1) = 0 Since k>0k > 0, we have −2k+1=0-2k + 1 = 0, so k=12k = \frac{1}{2}. This is choice (D). Choice (A) 2−22 - \sqrt{2} might tempt you if you incorrectly set up a quadratic formula problem or made algebraic errors. Choice (B) 11 could result from solving k2−k=0k^2 - k = 0 and choosing the wrong root, forgetting that we need the equation k2−k=3k2−2kk^2 - k = 3k^2 - 2k. Choice (C) 23\frac{2}{3} might come from inverting the correct answer or making sign errors in the algebraic manipulation. Remember: average value problems often lead to polynomial equations. Always set up the integral correctly, simplify the average value expression completely, then set it equal to the function value at the endpoint. Double-check your algebra—these problems are more about careful computation than complex concepts.
  10. What is the average value of the function f(x)=xx2+9f(x) = x\sqrt{x^2+9} on the interval [0,4][0, 4]?
    Answer: 49/649/6 The average value of a function over an interval is a fundamental concept in calculus that requires setting up and evaluating a definite integral. For any function f(x)f(x) on interval [a,b][a,b], the average value equals 1b−a∫abf(x) dx\frac{1}{b-a}\int_a^b f(x)\,dx. For f(x)=xx2+9f(x) = x\sqrt{x^2+9} on [0,4][0,4], you need to calculate 14−0∫04xx2+9 dx=14∫04xx2+9 dx\frac{1}{4-0}\int_0^4 x\sqrt{x^2+9}\,dx = \frac{1}{4}\int_0^4 x\sqrt{x^2+9}\,dx. To evaluate this integral, use substitution. Let u=x2+9u = x^2 + 9, so du=2x dxdu = 2x\,dx, which means x dx=12dux\,dx = \frac{1}{2}du. When x=0x = 0, u=9u = 9; when x=4x = 4, u=25u = 25. The integral becomes: 14⋅12∫925u du=18∫925u1/2 du=18⋅23u3/2∣925=112[253/2−93/2]=112[125−27]=9812=496\frac{1}{4} \cdot \frac{1}{2}\int_9^{25} \sqrt{u}\,du = \frac{1}{8}\int_9^{25} u^{1/2}\,du = \frac{1}{8} \cdot \frac{2}{3}u^{3/2}\Big|_9^{25} = \frac{1}{12}[25^{3/2} - 9^{3/2}] = \frac{1}{12}[125 - 27] = \frac{98}{12} = \frac{49}{6}. This confirms answer B is correct. Choice A (49/349/3) represents doubling the correct answer, likely from forgetting the factor of 12\frac{1}{2} in the substitution. Choice C (98/398/3) omits the 14\frac{1}{4} factor from the average value formula. Choice D (4/34/3) appears to confuse this with a different calculation entirely. Remember: average value problems always require both setting up the integral correctly AND applying the 1b−a\frac{1}{b-a} factor—don't forget either component.
  11. The temperature in degrees Celsius along a 3-meter metal rod, positioned from x=0x=0 to x=3x=3, is given by the function T(x)=90x(3−x)T(x) = 90x(3-x). What is the average temperature of the rod?
    Answer: 135∘C135^{\circ}C The average temperature is the average value of the function T(x)T(x) on [0,3][0, 3]. Tavg=13−0∫03(270x−90x2) dx=13[135x2−30x3]03=13[135(32)−30(33)]=13[1215−810]=4053=135T_{avg} = \frac{1}{3-0} \int_0^3 (270x - 90x^2) \,dx = \frac{1}{3} [135x^2 - 30x^3]_0^3 = \frac{1}{3} [135(3^2) - 30(3^3)] = \frac{1}{3}[1215 - 810] = \frac{405}{3} = 135. Distractor A is the value of the definite integral, forgetting to divide by the interval length. Distractor B is the temperature at the midpoint of the rod, T(1.5)T(1.5), which is also the maximum temperature, a common misconception for the average value.
  12. Let A(h)A(h) represent the average value of the function f(x)=exf(x) = e^x on the interval [2,2+h][2, 2+h]. Find lim⁡h→0A(h)\lim_{h \to 0} A(h).
    Answer: e2e^2 The average value is A(h)=1(2+h)−2∫22+hex dx=1h[ex]22+h=e2+h−e2hA(h) = \frac{1}{(2+h)-2} \int_2^{2+h} e^x \,dx = \frac{1}{h} [e^x]_2^{2+h} = \frac{e^{2+h} - e^2}{h}. The limit lim⁡h→0e2+h−e2h\lim_{h \to 0} \frac{e^{2+h} - e^2}{h} is the definition of the derivative of the function g(x)=exg(x) = e^x at x=2x=2. Since g′(x)=exg'(x) = e^x, the limit is g′(2)=e2g'(2) = e^2. Conceptually, as the interval shrinks, the average value of a continuous function approaches the function's value at that point.
  13. Let g(x)=∫1xt2 dtg(x) = \int_1^x t^2 \,dt. What is the average value of the function g(x)g(x) on the interval [1,4][1, 4]?
    Answer: 274\frac{27}{4} This problem combines two fundamental calculus concepts: the Fundamental Theorem of Calculus and average value of functions. When you see a function defined as an integral with a variable upper limit, you'll need to first find an explicit formula for that function. Start by evaluating the integral g(x)=∫1xt2 dtg(x) = \int_1^x t^2 \,dt. Using the antiderivative of t2t^2, which is t33\frac{t^3}{3}, you get: g(x)=[t33]1x=x33−133=x3−13g(x) = \left[\frac{t^3}{3}\right]_1^x = \frac{x^3}{3} - \frac{1^3}{3} = \frac{x^3 - 1}{3} Now find the average value using the formula: Average=1b−a∫abg(x) dx\text{Average} = \frac{1}{b-a}\int_a^b g(x)\,dx For the interval [1,4][1,4]: Average=14−1∫14x3−13 dx=19∫14(x3−1) dx\text{Average} = \frac{1}{4-1}\int_1^4 \frac{x^3-1}{3}\,dx = \frac{1}{9}\int_1^4 (x^3-1)\,dx =19[x44−x]14=19[(2564−4)−(14−1)]= \frac{1}{9}\left[\frac{x^4}{4} - x\right]_1^4 = \frac{1}{9}\left[\left(\frac{256}{4} - 4\right) - \left(\frac{1}{4} - 1\right)\right] =19(64−4−0.25+1)=19⋅2434=274= \frac{1}{9}(64 - 4 - 0.25 + 1) = \frac{1}{9} \cdot \frac{243}{4} = \frac{27}{4} Choice A is correct. Choice B (8512\frac{85}{12}) likely results from computational errors in the integration. Choice C (7) might come from forgetting the coefficient 13\frac{1}{3} in g(x)g(x). Choice D (2434\frac{243}{4}) is what you'd get if you forgot to divide by the interval length. Remember: when finding average values of functions defined by integrals, always evaluate the integral first to get an explicit function, then apply the average value formula carefully.
  14. Let ff be a continuous function and let FF be an antiderivative of ff. The average value of ff on the interval [a,b][a, b] is equivalent to which of the following quantities?
    Answer: The average rate of change of F(x)F(x) on [a,b][a, b]. When you see a question connecting average values with antiderivatives, think about how these concepts relate through the Fundamental Theorem of Calculus and the Mean Value Theorem. The average value of a function ff on [a,b][a,b] is defined as 1b−a∫abf(x) dx\frac{1}{b-a}\int_a^b f(x)\,dx. Since FF is an antiderivative of ff, the Fundamental Theorem tells us that ∫abf(x) dx=F(b)−F(a)\int_a^b f(x)\,dx = F(b) - F(a). Therefore, the average value becomes F(b)−F(a)b−a\frac{F(b) - F(a)}{b - a}. This expression is precisely the average rate of change of F(x)F(x) on [a,b][a,b] - the change in FF divided by the change in xx. So answer D is correct. Let's see why the other choices fail. Choice A suggests averaging the endpoint values of FF, but this ignores the behavior of FF throughout the interval and has no connection to integration. Choice B describes a specific point cc where f(c)f(c) equals the total change in FF, but this represents one particular value of ff, not its average over the entire interval. Choice C refers to the average rate of change of ff itself, which would be f(b)−f(a)b−a\frac{f(b) - f(a)}{b - a} - completely different from the average value of ff. Remember this key connection: the average value of a function equals the average rate of change of its antiderivative. This relationship appears frequently in calculus problems linking integration and differentiation.
  15. The function F(x)=x3−cos⁡(x)F(x) = x^3 - \cos(x) is an antiderivative of a function f(x)f(x). What is the average value of f(x)f(x) on the interval [0,π][0, \pi]?
    Answer: π2+2π\pi^2 + \frac{2}{\pi} This question combines two fundamental calculus concepts: the relationship between functions and their antiderivatives, and the formula for average value of a function. Since F(x)=x3−cos⁡(x)F(x) = x^3 - \cos(x) is an antiderivative of f(x)f(x), you need to find f(x)f(x) by taking the derivative: f(x)=F′(x)=3x2+sin⁡(x)f(x) = F'(x) = 3x^2 + \sin(x). Remember that the derivative of cos⁡(x)\cos(x) is −sin⁡(x)-\sin(x), so the derivative of −cos⁡(x)-\cos(x) is sin⁡(x)\sin(x). The average value of a function on interval [a,b][a,b] is 1b−a∫abf(x) dx\frac{1}{b-a}\int_a^b f(x)\,dx. Here, you need 1π−0∫0π(3x2+sin⁡(x)) dx=1π∫0π(3x2+sin⁡(x)) dx\frac{1}{\pi-0}\int_0^\pi (3x^2 + \sin(x))\,dx = \frac{1}{\pi}\int_0^\pi (3x^2 + \sin(x))\,dx. Since F(x)F(x) is an antiderivative of f(x)f(x), the Fundamental Theorem of Calculus gives us: ∫0πf(x) dx=F(π)−F(0)\int_0^\pi f(x)\,dx = F(\pi) - F(0). Calculate: F(π)=π3−cos⁡(π)=π3−(−1)=π3+1F(\pi) = \pi^3 - \cos(\pi) = \pi^3 - (-1) = \pi^3 + 1 and F(0)=03−cos⁡(0)=0−1=−1F(0) = 0^3 - \cos(0) = 0 - 1 = -1. So ∫0πf(x) dx=(π3+1)−(−1)=π3+2\int_0^\pi f(x)\,dx = (\pi^3 + 1) - (-1) = \pi^3 + 2. The average value is π3+2π=π2+2π\frac{\pi^3 + 2}{\pi} = \pi^2 + \frac{2}{\pi}, which is answer C. Answer A adds 1π\frac{1}{\pi} instead of 2π\frac{2}{\pi}, likely from miscalculating the definite integral. Answer B forgets the sin⁡(x)\sin(x) term entirely. Answer D gives the value of the definite integral itself, not the average value. Key strategy: When given an antiderivative, always differentiate first to find the original function, then use the Fundamental Theorem to evaluate definite integrals efficiently.
  16. Find the smallest positive value of cc in the interval [0,2π][0, 2\pi] such that f(c)f(c) equals the average value of f(x)=sin⁡(x)f(x) = \sin(x) on [0,2π][0, 2\pi].
    Answer: π\pi First, find the average value of sin⁡(x)\sin(x) on [0,2π][0, 2\pi]. favg=12π∫02πsin⁡(x) dx=12π[−cos⁡(x)]02π=12π(−cos⁡(2π)−(−cos⁡(0)))=12π(−1−(−1))=0f_{avg} = \frac{1}{2\pi} \int_0^{2\pi} \sin(x) \,dx = \frac{1}{2\pi} [-\cos(x)]_0^{2\pi} = \frac{1}{2\pi} (-\cos(2\pi) - (-\cos(0))) = \frac{1}{2\pi}(-1 - (-1)) = 0. Next, we must find values of cc in [0,2π][0, 2\pi] for which f(c)=0f(c) = 0, i.e., sin⁡(c)=0\sin(c) = 0. The solutions are c=0,π,2πc = 0, \pi, 2\pi. The smallest positive value among these is π\pi.
  17. For what value of k>0k > 0 is the average value of the function f(x)=xf(x) = \sqrt{x} on the interval [0,k][0, k] equal to 4?
    Answer: 3636 The average value is 1k−0∫0kx1/2 dx=1k[23x3/2]0k=1k(23k3/2)=23k1/2=2k3\frac{1}{k-0} \int_0^k x^{1/2} \,dx = \frac{1}{k} [\frac{2}{3}x^{3/2}]_0^k = \frac{1}{k} (\frac{2}{3}k^{3/2}) = \frac{2}{3}k^{1/2} = \frac{2\sqrt{k}}{3}. We set this equal to 4: 2k3=4  ⟹  2k=12  ⟹  k=6  ⟹  k=36\frac{2\sqrt{k}}{3} = 4 \implies 2\sqrt{k} = 12 \implies \sqrt{k} = 6 \implies k = 36. Distractor A is the value of k\sqrt{k}. Distractor B results from an error in the power rule for integration, using x3/2x^{3/2} as the antiderivative instead of 23x3/2\frac{2}{3}x^{3/2}.
  18. Let cc be a value in the interval [0,4][0, 4] guaranteed by the Mean Value Theorem for Integrals for the function f(x)=3x2−6xf(x) = 3x^2 - 6x. What is the value of cc?
    Answer: 1+2131 + \frac{\sqrt{21}}{3} First, find the average value of the function on the interval [0,4][0, 4]: favg=14−0∫04(3x2−6x) dx=14[x3−3x2]04=14(64−48)=164=4f_{avg} = \frac{1}{4-0} \int_0^4 (3x^2 - 6x) \,dx = \frac{1}{4} [x^3 - 3x^2]_0^4 = \frac{1}{4}(64 - 48) = \frac{16}{4} = 4. According to the Mean Value Theorem for Integrals, there exists a cc in [0,4][0, 4] such that f(c)=favgf(c) = f_{avg}. We must solve 3c2−6c=43c^2 - 6c = 4, which is 3c2−6c−4=03c^2 - 6c - 4 = 0. Using the quadratic formula, c=6±36−4(3)(−4)6=6±846=1±2216=1±213c = \frac{6 \pm \sqrt{36 - 4(3)(-4)}}{6} = \frac{6 \pm \sqrt{84}}{6} = 1 \pm \frac{2\sqrt{21}}{6} = 1 \pm \frac{\sqrt{21}}{3}. The two possible values are c≈1+1.53=2.53c \approx 1 + 1.53 = 2.53 and c≈1−1.53=−0.53c \approx 1 - 1.53 = -0.53. Only 1+2131 + \frac{\sqrt{21}}{3} is in the interval [0,4][0, 4].
  19. What is the average value of the function f(x)=cos⁡2(x)f(x) = \cos^2(x) on the interval [0,π/2][0, \pi/2]?
    Answer: 12\frac{1}{2} To find the average value, we compute 1π/2−0∫0π/2cos⁡2(x) dx\frac{1}{\pi/2 - 0} \int_0^{\pi/2} \cos^2(x) \,dx. We use the half-angle identity cos⁡2(x)=1+cos⁡(2x)2\cos^2(x) = \frac{1 + \cos(2x)}{2}. The expression becomes 2π∫0π/21+cos⁡(2x)2 dx=1π[x+12sin⁡(2x)]0π/2=1π[(π2+12sin⁡(π))−(0+12sin⁡(0))]=1π(π2)=12\frac{2}{\pi} \int_0^{\pi/2} \frac{1 + \cos(2x)}{2} \,dx = \frac{1}{\pi} [x + \frac{1}{2}\sin(2x)]_0^{\pi/2} = \frac{1}{\pi} [(\frac{\pi}{2} + \frac{1}{2}\sin(\pi)) - (0 + \frac{1}{2}\sin(0))] = \frac{1}{\pi}(\frac{\pi}{2}) = \frac{1}{2}. Distractor A is the value of the integral. Distractor B is the average value of cos⁡(x)\cos(x) on the interval.
  20. According to the Mean Value Theorem for Integrals, there exists a number cc in the interval [1,e][1, e] such that f(c)f(c) equals the average value of the function f(x)=1xf(x) = \frac{1}{x} on that interval. What is the value of this number cc?
    Answer: e−1e-1 The Mean Value Theorem for Integrals states that if ff is continuous on [a,b][a, b], there exists a cc in [a,b][a, b] such that f(c)=1b−a∫abf(x) dxf(c) = \frac{1}{b-a} \int_a^b f(x) \, dx. 1. First, calculate the average value of f(x)=1/xf(x) = 1/x on [1,e][1, e]: favg=1e−1∫1e1x dxf_{avg} = \frac{1}{e-1} \int_1^e \frac{1}{x} \, dx 2. The integral of 1/x1/x is ln⁡∣x∣\ln|x|: ∫1e1x dx=[ln⁡∣x∣]1e=ln⁡(e)−ln⁡(1)=1−0=1\int_1^e \frac{1}{x} \, dx = [\ln|x|]_1^e = \ln(e) - \ln(1) = 1 - 0 = 1 3. Substitute this back into the average value formula: favg=1e−1⋅1=1e−1f_{avg} = \frac{1}{e-1} \cdot 1 = \frac{1}{e-1} 4. Now, find the value of cc such that f(c)=favgf(c) = f_{avg}: f(c)=1c=1e−1f(c) = \frac{1}{c} = \frac{1}{e-1} 5. Solving for cc gives: c=e−1c = e-1 Distractor A is the average value of the function, favgf_{avg}, not the value of cc. Distractor C is the midpoint of the interval, a common but incorrect guess. Distractor D is the value of the definite integral, ∫1ef(x)dx\int_1^e f(x) dx, without dividing by the interval length.

Start this quiz