Fundamental Theorem of Calculus: Accumulation Functions

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AP Calculus AB › Fundamental Theorem of Calculus: Accumulation Functions

Questions 1 - 10
1

Suppose $K(x)=\int_{0}^{x}\big(2^{t}+\cos t\big),dt$. What is $K'(x)$?

$\left[\dfrac{2^{t}}{\ln 2}+\sin t\right]_{0}^{x}$

$2^{x}\ln 2-\sin x$

$2^{x}+\cos x$

$2^{t}+\cos t$

$\int_{0}^{x}\big(2^{t}+\cos t\big),dt$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1 to find the derivative of an accumulation function. The function K(x) is defined as the integral from 0 to x of (2ᵗ + cos t) dt, which accumulates the area under the curve of f(t) = 2ᵗ + cos t from a fixed lower limit to the variable upper limit x. According to FTC1, the derivative K'(x) equals the integrand evaluated at x, which is 2ˣ + cos x. This result holds because the instantaneous rate of change of the accumulated integral at x is precisely the value of the function being integrated at that point. A tempting distractor is choice E, which represents the evaluation of an antiderivative from 0 to x, but that actually computes K(x) itself, not its derivative. To recognize FTC Part 1 problems, look for functions defined as definite integrals with a variable upper limit and a fixed lower limit, and questions asking for the derivative of that function.

2

Let $C(x)=\int_{-5}^{x} \big| t \big| , dt$. What is $C'(x)$?

$|t|$

$\left[\dfrac{t|t|}{2}\right]_{-5}^{x}$

$|x|$

$\dfrac{x}{|x|}$

$\int_{-5}^{x}|t|,dt$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1 to find the derivative of an accumulation function. The function C(x) is defined as the integral from -5 to x of $|t|$ dt, which accumulates the area under the curve of f(t) = $|t|$ from a fixed lower limit to the variable upper limit x. According to FTC1, the derivative C'(x) equals the integrand evaluated at x, which is $|x|$. This result holds because the instantaneous rate of change of the accumulated integral at x is precisely the value of the function being integrated at that point. A tempting distractor is choice E, which represents the evaluation of an antiderivative from -5 to x, but that actually computes C(x) itself, not its derivative. To recognize FTC Part 1 problems, look for functions defined as definite integrals with a variable upper limit and a fixed lower limit, and questions asking for the derivative of that function.

3

Define $H(x)=\int_{-1}^{x}(t-1)^5,dt$. What is $H'(x)$?

$(t-1)^5$

$\int_{-1}^{x}(t-1)^5,dt$

$\left[\dfrac{(t-1)^6}{6}\right]_{-1}^{x}$

$(x-1)^5$

$5(x-1)^4$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1 to find the derivative of an accumulation function. The function H(x) is defined as the integral from -1 to x of (t - 1)⁵ dt, which accumulates the area under the curve of f(t) = (t - 1)⁵ from a fixed lower limit to the variable upper limit x. According to FTC1, the derivative H'(x) equals the integrand evaluated at x, which is (x - 1)⁵. This result holds because the instantaneous rate of change of the accumulated integral at x is precisely the value of the function being integrated at that point. A tempting distractor is choice E, which represents the evaluation of an antiderivative from -1 to x, but that actually computes H(x) itself, not its derivative. To recognize FTC Part 1 problems, look for functions defined as definite integrals with a variable upper limit and a fixed lower limit, and questions asking for the derivative of that function.

4

Suppose $S(x)=\int_{3}^{x}\big(\cos t+\dfrac{1}{t^2}\big),dt$. What is $S'(x)$?

$-\sin x-\dfrac{2}{x^3}$

$\cos t+\dfrac{1}{t^2}$

$\left[\sin t-\dfrac{1}{t}\right]_{3}^{x}$

$\cos x+\dfrac{1}{x^2}$

$\int_{3}^{x}\big(\cos t+\dfrac{1}{t^2}\big),dt$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1 to find the derivative of an accumulation function. The function S(x) is defined as the integral from 3 to x of (cos t + $1/t^2$) dt, which accumulates the area under the curve of the integrand from a fixed lower limit to a variable upper limit x. According to FTC Part 1, the derivative S'(x) is simply the integrand evaluated at t = x, so S'(x) = cos x + $1/x^2$. This holds because differentiating the integral with respect to the upper limit effectively adds the integrand's value at that point, reversing the integration process. A tempting distractor is choice B, which is actually the derivative of choice A and might be selected if someone mistakenly differentiates the result further. To recognize FTC Part 1 applications, look for functions defined as integrals with a variable upper limit and constant lower limit, where the derivative is the integrand at x.

5

Let $P(x)=\int_{-2}^{x}\dfrac{1}{\sqrt{t+3}},dt$ for $x>-3$. What is $P'(x)$?

$\dfrac{-1}{2(x+3)^{3/2}}$

$\int_{-2}^{x}\dfrac{1}{\sqrt{t+3}},dt$

$\dfrac{1}{\sqrt{x+3}}$

$\left[2\sqrt{t+3}\right]_{-2}^{x}$

$\dfrac{1}{\sqrt{t+3}}$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1 to find the derivative of an accumulation function. The function P(x) is defined as the integral from -2 to x of 1/sqrt(t+3) dt, which accumulates the area under the curve of the integrand from a fixed lower limit to a variable upper limit x. According to FTC Part 1, the derivative P'(x) is simply the integrand evaluated at t = x, so P'(x) = 1/sqrt(x+3). This holds because differentiating the integral with respect to the upper limit effectively adds the integrand's value at that point, reversing the integration process. A tempting distractor is choice B, which is actually the derivative of choice A and might be selected if someone mistakenly differentiates the result further. To recognize FTC Part 1 applications, look for functions defined as integrals with a variable upper limit and constant lower limit, where the derivative is the integrand at x.

6

Let $b(x)=\int_{0}^{x}\big(\sec t\big),dt$ on an interval where $\sec t$ is defined. What is $b'(x)$?

$\sec x\tan x$

$\sec t$

$\left[\ln|\sec t+\tan t|\right]_{0}^{x}$

$\int_{0}^{x}\sec t,dt$

$\sec x$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1 to find the derivative of an accumulation function. The function b(x) is defined as the $\int_0^x \sec t , dt$, which accumulates the area under the curve of the integrand from a fixed lower limit to a variable upper limit x. According to FTC Part 1, the derivative b'(x) is simply the integrand evaluated at t = x, so $b'(x) = \sec x$. This holds because differentiating the integral with respect to the upper limit effectively adds the integrand's value at that point, reversing the integration process. A tempting distractor is choice B, which is actually the derivative of choice A and might be selected if someone mistakenly differentiates the result further. To recognize FTC Part 1 applications, look for functions defined as integrals with a variable upper limit and constant lower limit, where the derivative is the integrand at x.

7

Let $X(x)=\int_{-2}^{x}\dfrac{1}{(t^2+1)^{1/2}},dt$. What is $X'(x)$?

$\int_{-2}^{x}\dfrac{1}{(t^2+1)^{1/2}},dt$

$\dfrac{-x}{(x^2+1)^{3/2}}$

$\dfrac{1}{\sqrt{x^2+1}}$

$\left[\ln\left|t+\sqrt{t^2+1}\right|\right]_{-2}^{x}$

$\dfrac{1}{\sqrt{t^2+1}}$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1 to find the derivative of an accumulation function. The function X(x) is defined as the integral from -2 to x of $\frac{1}{\sqrt{t^2+1}}$ dt, which accumulates the area under the curve of the integrand from a fixed lower limit to a variable upper limit x. According to FTC Part 1, the derivative X'(x) is simply the integrand evaluated at t = x, so X'(x) = $\frac{1}{\sqrt{x^2+1}}$. This holds because differentiating the integral with respect to the upper limit effectively adds the integrand's value at that point, reversing the integration process. A tempting distractor is choice B, which is actually the derivative of choice A and might be selected if someone mistakenly differentiates the result further. To recognize FTC Part 1 applications, look for functions defined as integrals with a variable upper limit and constant lower limit, where the derivative is the integrand at x.

8

Let $g(x)=\int_{5}^{x}\frac{1}{1+t^2},dt$. What is $g'(x)$?

$\dfrac{1}{1+x^2}$

$\displaystyle \int_{5}^{x}\frac{1}{1+t^2},dt$

$\displaystyle \frac{1}{1+t^2}\big|_{t=5}$

$\displaystyle \int_{5}^{x}\frac{-2t}{(1+t^2)^2},dt$

$\dfrac{-2x}{(1+x^2)^2}$

Explanation

This is a direct application of the Fundamental Theorem of Calculus Part 1, which tells us that the derivative of an accumulation function equals the integrand evaluated at the variable limit. For $g(x) = \int_{5}^{x}\frac{1}{1+t^2},dt$, we get $g'(x) = \frac{1}{1+x^2}$ by substituting x for t in the integrand. The process doesn't require finding an antiderivative or performing any integration. Choice E ($\frac{-2x}{(1+x^2)^2}$) represents the derivative of the integrand function, which would be incorrect since FTC Part 1 calls for evaluation, not differentiation. To master FTC Part 1, remember that $\frac{d}{dx}\int_{a}^{x}f(t),dt = f(x)$ when a is constant.

9

Let $G(x)=\int_{5}^{x}\dfrac{1}{1+t^2},dt$. What is $G'(x)$?​​

$\dfrac{1}{1+x^2}$

$\dfrac{-1}{1+x^2}$

$\arctan(x)-\arctan(5)$

$\displaystyle \int_{5}^{x}\dfrac{1}{1+t^2},dt$

$\dfrac{1}{1+5^2}$

Explanation

This is a direct application of the Fundamental Theorem of Calculus Part 1, which states that the derivative of $\int_{a}^{x}f(t),dt$ equals $f(x)$. Given $G(x) = \int_{5}^{x}\frac{1}{1+t^2},dt$, we find $G'(x)$ by evaluating the integrand at the upper limit $x$. This gives us $G'(x) = \frac{1}{1+x^2}$, obtained by replacing $t$ with $x$ in the integrand. Choice E ($\arctan(x)-\arctan(5)$) might tempt students who recognize that $\int\frac{1}{1+t^2},dt = \arctan(t)+C$, but this is the antiderivative, not the derivative we seek. Remember: when differentiating an accumulation function, simply evaluate the integrand at the variable upper limit.

10

If $T(x)=\int_{-3}^{x}\left(e^{2t}-\cos t\right)dt$, what is $T'(x)$?

$\displaystyle \int_{-3}^{x}\left(e^{2t}-\cos t\right)dt$

$\displaystyle(e^{2t}-\cos t)\big|_{t=-3}$

$\displaystyle \int_{-3}^{x}\left(2e^{2t}+\sin t\right)dt$

$2e^{2x}+\sin x$

$e^{2x}-\cos x$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1, where the derivative of an integral with variable upper limit equals the integrand evaluated at that limit. Given $T(x) = \int_{-3}^{x}(e^{2t} - \cos t),dt$, we get $T'(x) = e^{2x} - \cos x$ by replacing t with x in the integrand. The process is purely evaluation; no integration or differentiation of the integrand is performed. Choice A ($2e^{2x} + \sin x$) shows the derivatives of $e^{2x}$ and $-\cos x$, which would result from incorrectly differentiating after applying FTC Part 1. To master this concept, remember that FTC Part 1 converts "derivative of integral" problems into simple substitution exercises.

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