Fundamental Theorem of Calculus: Accumulation Functions

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AP Calculus BC › Fundamental Theorem of Calculus: Accumulation Functions

Questions 1 - 10
1

A car’s acceleration is $a(t)=\sqrt{t+4}$. If $V(x)=\int_{2}^{x} a(t),dt$, what is $V'(x)$?

$\int_{2}^{x} \sqrt{x+4},dt$

$\int_{2}^{x} \sqrt{t+4},dx$

$\int_{2}^{x} \sqrt{t+4},dt$

$\sqrt{x+4}$

$\sqrt{t+4}$

Explanation

This problem requires applying the first part of the Fundamental Theorem of Calculus (FTC Part 1), which states that if V(x) is defined as the integral from a constant to x of a(t) dt, then V'(x) equals a(x). Here, V(x) accumulates the acceleration a(t) = √(t+4) from t=2 to t=x, so its derivative V'(x) should simply be the integrand evaluated at x, which is √(x+4). FTC Part 1 works because the derivative of the accumulated velocity up to x is precisely the acceleration at that point x. This is like how the rate of change of speed at time x is exactly the acceleration at that instant. A tempting distractor is choice D, √(t+4), but it fails because it doesn't replace the dummy variable t with the upper limit x, leaving it in terms of t instead of expressing the derivative as a function of x. To recognize FTC Part 1 problems, look for a function defined as an integral with a variable upper limit and a request for its derivative, which will always be the integrand with t swapped for x.

2

A machine’s output rate is $q(t)=\ln(t+5)$. Let $O(x)=\int_{0}^{x} q(t),dt$. What is $O'(x)$?

$\int_{0}^{x} \ln(t+5),dx$

$\ln(t+5)$

$\ln(x+5)$

$\int_{0}^{x} \ln(t+5),dt$

$\int_{0}^{x} \ln(x+5),dt$

Explanation

This problem requires applying the first part of the Fundamental Theorem of Calculus (FTC Part 1), which states that if O(x) is defined as the integral from a constant to x of q(t) dt, then O'(x) equals q(x). Here, O(x) accumulates the output rate q(t) = ln(t+5) from t=0 to t=x, so its derivative O'(x) should simply be the integrand evaluated at x, which is ln(x+5). FTC Part 1 works because the derivative of the accumulated output up to x is precisely the output rate at that point x. This is like how the rate of change of total output at time x is exactly the production rate at that instant. A tempting distractor is choice C, ln(t+5), but it fails because it doesn't replace the dummy variable t with the upper limit x, leaving it in terms of t instead of expressing the derivative as a function of x. To recognize FTC Part 1 problems, look for a function defined as an integral with a variable upper limit and a request for its derivative, which will always be the integrand with t swapped for x.

3

A spring’s force varies as $F(t)=t^3-2$. If $W(x)=\int_{-2}^{x} F(t),dt$, what is $W'(x)$?

$x^3-2$

$t^3-2$

$\int_{-2}^{x} (t^3-2),dt$

$\int_{-2}^{x} (x^3-2),dt$

$\int_{-2}^{x} (t^3-2),dx$

Explanation

This problem requires applying the first part of the Fundamental Theorem of Calculus (FTC Part 1), which states that if W(x) is defined as the integral from a constant to x of F(t) dt, then W'(x) equals F(x). Here, W(x) accumulates the force F(t) = t³ - 2 from t=-2 to t=x, so its derivative W'(x) should simply be the integrand evaluated at x, which is x³ - 2. FTC Part 1 works because the derivative of the accumulated work up to x is precisely the force at that point x. This is like how the rate of change of total work at position x is exactly the force at that instant. A tempting distractor is choice B, t³ - 2, but it fails because it doesn't replace the dummy variable t with the upper limit x, leaving it in terms of t instead of expressing the derivative as a function of x. To recognize FTC Part 1 problems, look for a function defined as an integral with a variable upper limit and a request for its derivative, which will always be the integrand with t swapped for x.

4

A runner’s speed is $v(t)=\sin t+t$. Let $S(x)=\int_{0}^{x} v(t),dt$. What is $S'(x)$?

$\sin t+t$

$\int_{0}^{x} (\sin x+x),dt$

$\int_{0}^{x} (\sin t+t),dx$

$\int_{0}^{x} (\sin t+t),dt$

$\sin x+x$

Explanation

This problem requires applying the first part of the Fundamental Theorem of Calculus (FTC Part 1), which states that if S(x) is defined as the integral from a constant to x of v(t) dt, then S'(x) equals v(x). Here, S(x) accumulates the speed v(t) = sin t + t from t=0 to t=x, so its derivative S'(x) should simply be the integrand evaluated at x, which is sin x + x. FTC Part 1 works because the derivative of the accumulated distance up to x is precisely the speed at that point x. This is like how the rate of change of position at time x is exactly the velocity at that instant. A tempting distractor is choice D, sin t + t, but it fails because it doesn't replace the dummy variable t with the upper limit x, leaving it in terms of t instead of expressing the derivative as a function of x. To recognize FTC Part 1 problems, look for a function defined as an integral with a variable upper limit and a request for its derivative, which will always be the integrand with t swapped for x.

5

A bacteria culture grows at rate $g(t)=e^{2t}$. If $P(x)=\int_{-1}^{x} g(t),dt$, what is $P'(x)$?

$\int_{-1}^{x} e^{2x},dt$

$e^{2t}$

$e^{2x}$

$\int_{-1}^{x} e^{2t},dt$

$\int_{-1}^{x} e^{2t},dx$

Explanation

This problem requires applying the first part of the Fundamental Theorem of Calculus (FTC Part 1), which states that if $P(x)$ is defined as the integral from a constant to x of $g(t) , dt$, then $P'(x)$ equals $g(x)$. Here, $P(x)$ accumulates the growth rate $g(t) = e^{2t}$ from t=-1 to t=x, so its derivative $P'(x)$ should simply be the integrand evaluated at x, which is $e^{2x}$. FTC Part 1 works because the derivative of the accumulated population up to x is precisely the growth rate at that point x. This is like how the rate of change of total bacteria at time x is exactly the growth rate at that instant. A tempting distractor is choice B, $e^{2t}$, but it fails because it doesn't replace the dummy variable t with the upper limit x, leaving it in terms of t instead of expressing the derivative as a function of x. To recognize FTC Part 1 problems, look for a function defined as an integral with a variable upper limit and a request for its derivative, which will always be the integrand with t swapped for x.

6

A tank’s net inflow rate is $r(t)=t^2+3t$. If $A(x)=\int_{1}^{x} r(t),dt$, what is $A'(x)$?

$\int_{1}^{x} (x^2+3x),dt$

$t^2+3t$

$\int_{1}^{x} (t^2+3t),dx$

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

$x^2+3x$

Explanation

This problem requires applying the first part of the Fundamental Theorem of Calculus (FTC Part 1), which states that if A(x) is defined as the integral from a constant to x of r(t) dt, then A'(x) equals r(x). Here, A(x) accumulates the net inflow rate r(t) = t² + 3t from t=1 to t=x, so its derivative A'(x) should simply be the integrand evaluated at x, which is x² + 3x. FTC Part 1 works because the derivative of the accumulated area under the curve r(t) up to x is precisely the height of the function at that point x. This is like how the rate of change of total water in the tank at time x is exactly the inflow rate at that instant. A tempting distractor is choice E, t² + 3t, but it fails because it doesn't replace the dummy variable t with the upper limit x, leaving it in terms of t instead of expressing the derivative as a function of x. To recognize FTC Part 1 problems, look for a function defined as an integral with a variable upper limit and a request for its derivative, which will always be the integrand with t swapped for x.

7

Let $F(x)=\int_{-2}^x \frac{1}{1+t^4},dt$. What is $F'(x)$?

$\displaystyle \frac{1}{1+t^4}$

$\displaystyle \frac{4x^3}{(1+x^4)^2}$

$\displaystyle \int_{-2}^x \frac{1}{1+t^4},dt$

$\displaystyle \frac{1}{1+x^4}$

$\displaystyle \frac{1}{1+(-2)^4}$

Explanation

This problem demonstrates the Fundamental Theorem of Calculus Part 1, which connects accumulation functions to their rate functions. Since F(x) = $\int_{-2}^x \frac{1}{1+t^4} , dt$, applying FTC Part 1 gives us F'(x) = $\frac{1}{1+x^4}$. The theorem states that differentiating an integral with respect to its upper limit yields the integrand evaluated at that upper limit. We replace the dummy variable t with x to get our answer. Choice B incorrectly keeps the dummy variable t, while choice D mistakenly tries to differentiate the integrand using the chain rule. To apply FTC Part 1 correctly, identify that x appears only as an integration limit, then substitute x for the dummy variable in the integrand.

8

If $G(x)=\int_5^x (3t-4)e^{t},dt$ models accumulated growth, what is $G'(x)$?

$\displaystyle 3e^{x}$

$\displaystyle(3x-4)e^{5}$

$\displaystyle(3x-4)e^{x}$

$\displaystyle(3t-4)e^{t}$

$\displaystyle \int_5^x (3t-4)e^{t},dt$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1 to find the instantaneous rate of growth. Given G(x) = ∫₅ˣ (3t-4)eᵗ dt, the FTC Part 1 tells us that G'(x) equals the integrand evaluated at x, which gives us (3x-4)eˣ. The derivative of an accumulation function is simply the integrand with the dummy variable t replaced by the upper limit x. Choice C incorrectly retains the dummy variable t, failing to evaluate at the upper limit. Choice D attempts to differentiate the integrand rather than evaluate it. When you see a derivative of an integral where x appears only in the limits, apply FTC Part 1 by substituting the upper limit into the integrand.

9

A particle’s velocity is $v(t)=t^2\sin t$. If $A(x)=\int_0^x v(t),dt$, what is $A'(x)$?

$\displaystyle \int_0^x t^2\sin t,dt$

$\displaystyle x^2\cos x$

$\displaystyle x^2\sin x$

$\displaystyle x\sin x$

$\displaystyle \sin x$

Explanation

This problem requires applying the Fundamental Theorem of Calculus Part 1, which states that the derivative of an accumulation function equals the integrand evaluated at the upper limit. Since A(x) = ∫₀ˣ v(t) dt where v(t) = t²sin t, we have A'(x) = v(x) = x²sin x. The FTC Part 1 tells us that when we differentiate an integral with respect to its upper limit, we simply substitute that upper limit into the integrand. Choice A incorrectly suggests the derivative is the integral itself, failing to apply the FTC. To recognize FTC Part 1 problems, look for derivatives of integrals where the variable appears only in the limits of integration.

10

Define $P(x)=\int_{3}^{x}\frac{t}{\sqrt{t^2+9}},dt$ as accumulated work. What is $P'(x)$?

$\frac{3}{\sqrt{3^2+9}}$

$\int_{3}^{x}\frac{x}{\sqrt{x^2+9}},dt$

$\frac{x}{\sqrt{x^2+9}}$

$\int_{3}^{x}\frac{9}{(t^2+9)^{3/2}},dt$

$\int_{3}^{x}\frac{t}{\sqrt{t^2+9}},dt$

Explanation

This problem requires applying the first part of the Fundamental Theorem of Calculus (FTC Part 1) to find the derivative of an accumulation function. The FTC Part 1 states that if P(x) = ∫ from a constant lower limit to x of f(t) dt, then P'(x) = f(x), provided f is continuous at x. For this, f(t) = t / √(t² + 9), so P'(x) = x / √(x² + 9). This principle shows the derivative is the integrand substituted with x. A tempting distractor is choice E, which integrates an expression resembling the derivative of the integrand, but that confuses FTC with unnecessary differentiation. To recognize FTC Part 1 problems, look for questions asking for the derivative of a function defined as a definite integral with a variable upper limit.

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