AP Calculus AB : Use of the Fundamental Theorem to represent a particular antiderivative, and the analytical and graphical analysis of functions so defined

Study concepts, example questions & explanations for AP Calculus AB

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Example Questions

Example Question #1 : Integrals

Evaluate \(\displaystyle \frac{d}{dx}\int_4^{x^2} e^{t^2}dt\).

Possible Answers:

\(\displaystyle \frac{e^{t^2}}{t^2}\)

\(\displaystyle 2xe^{x^4}\)

Does not exist

\(\displaystyle x^2e^{x^2}\)

\(\displaystyle e^{x^2}\)

Correct answer:

\(\displaystyle 2xe^{x^4}\)

Explanation:

Even though an antideritvative of \(\displaystyle e^{t^2}\) does not exist, we can still use the Fundamental Theorem of Calculus to "cancel out" the integral sign in this expression.

 

\(\displaystyle \frac{d}{dx}\int_4^{x^2} e^{t^2}dt\). Start

\(\displaystyle = e^{(x^2)^2} \times (x^2)'\). You can "cancel out" the integral sign with the derivative by making sure the lower bound of the integral is a constant, the upper bound is a differentiable function of \(\displaystyle x\), \(\displaystyle f(x)\), and then substituting \(\displaystyle t = f(x)\) in the integrand. Lastly the Theorem states you must multiply your result by \(\displaystyle f'(x)\) (similar to the directions in using the chain rule).

\(\displaystyle =2xe^{x^4}\).

Example Question #2 : Integrals

The graph of a function \(\displaystyle f(x)\) is drawn below. Select the best answers to the following: 

 

Pbstm

 

\(\displaystyle 1)\) What is the best interpretation of the function?

 \(\displaystyle F(x)=\int_a^xf(t)dt\)

 

\(\displaystyle 2)\) Which plot shows the derivative of the function \(\displaystyle F(x)\)?

 

 

 

 

Possible Answers:

Wrong3q10

Wrn4

Wrngan2

Question 10 correct answer

Correct answer:

Question 10 correct answer

Explanation:

\(\displaystyle 1)\)   \(\displaystyle F(x)=\int_a^xf(t)dt\)

 

The function \(\displaystyle F(x)\) represents the area under the curve \(\displaystyle f(x)\) from \(\displaystyle x=a\) to some value of \(\displaystyle x>a\).  

 

Do not be confused by the use of \(\displaystyle t\) in the integrand. The reason we use \(\displaystyle t\) is because are writing the area as a function of \(\displaystyle x\), which requires that we treat the upper limit of integration as a variable \(\displaystyle x\). So we replace the independent variable of \(\displaystyle f(x)\) with a dummy index \(\displaystyle t\) when we write down the integral. It does not change the fundamental behavior of the function \(\displaystyle F(x)\) or \(\displaystyle f(t)\)

 

 \(\displaystyle 2)\) The graph of the derivative of \(\displaystyle F(x)\) is the same as the graph for \(\displaystyle f(x)\). This follows directly from the Second Fundamental Theorem of Calculus.

If the function \(\displaystyle f(x)\) is continuous on an interval \(\displaystyle I\) containing \(\displaystyle a\), then the function defined by: 

 \(\displaystyle F(x)=\int_a^xf(t)dt\)

has for its' derivative \(\displaystyle F'(x)=f(x)\)

 

 

Example Question #3 : Integrals

Evaluate 

\(\displaystyle \int_{-(4\pi+e^4)}^{4\pi+e^4}3\sin(2x)dx\)

Possible Answers:

\(\displaystyle 1\)

\(\displaystyle \frac{-3\cos(8\pi+2e^4)}{2}+\frac{3\cos(-8\pi-2e^4)}{2}\)

\(\displaystyle \frac{-3\cos(8\pi+2e^4)}{2}\)

\(\displaystyle \frac{-3\cos(\pi+e^4)}{2}\)

\(\displaystyle 0\)

Correct answer:

\(\displaystyle 0\)

Explanation:

Here we could use the Fundamental Theorem of Calculus to evaluate the definite integral; however, that might be difficult and messy.

Instead, we make a clever observation of the graph of

\(\displaystyle 3\sin(2x)\)

Namely, that

\(\displaystyle 3\sin(2x)=-3\sin(-2x)\)

This means that the values of the graph when comparing x and -x are equal but opposite. Then we can conclude that

\(\displaystyle \int_{-(4\pi+e^4)}^{4\pi+e^4}3\sin(2x)dx=\int_{-(4\pi+e^4)}^{0}3\sin(2x)dx+\int_{0}^{4\pi+e^4}3\sin(2x)dx\)

\(\displaystyle =-(SOMETHING)+(SOMETHING)=0\)

 

 

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