AP Calculus AB : Derivative at a point

Study concepts, example questions & explanations for AP Calculus AB

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Example Questions

Example Question #135 : Derivatives

Find the slope of the line tangent to the function 

\(\displaystyle g(x)=x^2\cos(x)\)

at \(\displaystyle x=\pi\)

Possible Answers:

\(\displaystyle 0\)

\(\displaystyle 1\)

\(\displaystyle -2\pi\)

\(\displaystyle \pi\)

\(\displaystyle 2\pi\)

Correct answer:

\(\displaystyle -2\pi\)

Explanation:

The slope of the line tangent to a function is nothing more than the first derivative, which is for this function equal to

\(\displaystyle g'(x)=2x\cos(x)-x^2\sin(x)\)

found using the following rules:

\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} f(x)g(x)=f'(x)g(x)+f(x)g'(x)\)\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} \cos(x)=-\sin(x)\)\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} x^n=nx^{n-1}\)

Evaluated at the given point, we find the slope of the tangent line equal to

\(\displaystyle g'(\pi)=2\pi(\cos(\pi))-\pi^2(\sin(\pi))=-2\pi\)

Example Question #136 : Derivatives

Functionplot

\(\displaystyle \begin{align*}&\text{Determine the sign of the derivative of the function above at the point }\\&x=2\end{align*}\)

Possible Answers:

\(\displaystyle \text{The derivative is zero.}\)

\(\displaystyle \text{The derivative is negative.}\)

\(\displaystyle \text{The derivative is positive.}\)

Correct answer:

\(\displaystyle \text{The derivative is positive.}\)

Explanation:

\(\displaystyle \begin{align*}&\text{The derivative of a function at a given point is synonymous}\\&\text{with the slope of a function at a given point. Therefore if}\\&\text{the function increases in value as x increases across a point,}\\&\text{the slope and derivative are positive. Conversely, if the function}\\&\text{decreases in value as x increases across a point, both slope}\\&\text{and derivative are negative. At points where the function is}\\&\text{flat, the slope and derivative are zero.}\\&\text{Studying the slope at }x=2\text{, the function appears to be increasing.}\\&\text{The derivative is positive.}\end{align*}\)

Example Question #142 : Ap Calculus Ab

Functionplot

\(\displaystyle \begin{align*}&\text{Determine the sign of the derivative of the function above at the point }\\&x=4\end{align*}\)

Possible Answers:

\(\displaystyle \text{The derivative is negative.}\)

\(\displaystyle \text{The derivative is positive.}\)

\(\displaystyle \text{The derivative is zero.}\)

Correct answer:

\(\displaystyle \text{The derivative is negative.}\)

Explanation:

\(\displaystyle \begin{align*}&\text{The derivative of a function at a given point is synonymous}\\&\text{with the slope of a function at a given point. Therefore if}\\&\text{the function increases in value as x increases across a point,}\\&\text{the slope and derivative are positive. Conversely, if the function}\\&\text{decreases in value as x increases across a point, both slope}\\&\text{and derivative are negative. At points where the function is}\\&\text{flat, the slope and derivative are zero.}\\&\text{Studying the slope at }x=4\text{, the function appears to be decreasing.}\\&\text{The derivative is negative.}\end{align*}\)

Example Question #143 : Ap Calculus Ab

Functionplot

\(\displaystyle \begin{align*}&\text{Determine the sign of the derivative of the function above at the point }\\&x=3\end{align*}\)

Possible Answers:

\(\displaystyle \text{The derivative is zero.}\)

\(\displaystyle \text{The derivative is positive.}\)

\(\displaystyle \text{The derivative is negative.}\)

Correct answer:

\(\displaystyle \text{The derivative is positive.}\)

Explanation:

\(\displaystyle \begin{align*}&\text{The derivative of a function at a given point is synonymous}\\&\text{with the slope of a function at a given point. Therefore if}\\&\text{the function increases in value as x increases across a point,}\\&\text{the slope and derivative are positive. Conversely, if the function}\\&\text{decreases in value as x increases across a point, both slope}\\&\text{and derivative are negative. At points where the function is}\\&\text{flat, the slope and derivative are zero.}\\&\text{Studying the slope at }x=3\text{, the function appears to be increasing.}\\&\text{The derivative is positive.}\end{align*}\)

Example Question #144 : Ap Calculus Ab

Functionplot

\(\displaystyle \begin{align*}&\text{Determine the sign of the derivative of the function above at the point }\\&x=-0.3\end{align*}\)

Possible Answers:

\(\displaystyle \text{The derivative is negative.}\)

\(\displaystyle \text{The derivative is positive.}\)

\(\displaystyle \text{The derivative is zero}\)

Correct answer:

\(\displaystyle \text{The derivative is zero}\)

Explanation:

\(\displaystyle \begin{align*}&\text{The derivative of a function at a given point is synonymous}\\&\text{with the slope of a function at a given point. Therefore if}\\&\text{the function increases in value as x increases across a point,}\\&\text{the slope and derivative are positive. Conversely, if the function}\\&\text{decreases in value as x increases across a point, both slope}\\&\text{and derivative are negative. At points where the function is}\\&\text{flat, the slope and derivative are zero.}\\&\text{Studying the slope at }x=-0.3\text{, the function appears to be flat.}\\&\text{The derivative is zero}\end{align*}\)

Example Question #145 : Ap Calculus Ab

Functionplot

\(\displaystyle \begin{align*}&\text{Determine the sign of the derivative of the function above at the point }\\&x=0.2\end{align*}\)

Possible Answers:

\(\displaystyle \text{The derivative is positive.}\)

\(\displaystyle \text{The derivative is zero}\)

\(\displaystyle \text{The derivative is negative.}\)

Correct answer:

\(\displaystyle \text{The derivative is zero}\)

Explanation:

\(\displaystyle \begin{align*}&\text{The derivative of a function at a given point is synonymous}\\&\text{with the slope of a function at a given point. Therefore if}\\&\text{the function increases in value as x increases across a point,}\\&\text{the slope and derivative are positive. Conversely, if the function}\\&\text{decreases in value as x increases across a point, both slope}\\&\text{and derivative are negative. At points where the function is}\\&\text{flat, the slope and derivative are zero.}\\&\text{Studying the slope at }x=0.2\text{, the function appears to be flat.}\\&\text{The derivative is zero}\end{align*}\)

Example Question #31 : Slope Of A Curve At A Point

Functionplot

\(\displaystyle \begin{align*}&\text{Determine whether or not the derivative of the function above at the point }\\&x=-1\\&\text{is positive, negative, or zero.}\end{align*}\)

Possible Answers:

\(\displaystyle \text{The derivative is zero.}\)

\(\displaystyle \text{The derivative is positive.}\)

\(\displaystyle \text{The derivative is negative.}\)

Correct answer:

\(\displaystyle \text{The derivative is negative.}\)

Explanation:

\(\displaystyle \begin{align*}&\text{The derivative of a function at a given point is synonymous}\\&\text{with the slope of a function at a given point. Therefore if}\\&\text{the function increases in value as x increases across a point,}\\&\text{the slope and derivative are positive. Conversely, if the function}\\&\text{decreases in value as x increases across a point, both slope}\\&\text{and derivative are negative. At points where the function is}\\&\text{flat, the slope and derivative are zero.}\\&\text{Studying the slope at }x=-1\text{, the function appears to be decreasing.}\\&\text{The derivative is negative.}\end{align*}\)

Example Question #32 : Slope Of A Curve At A Point

Functionplot

\(\displaystyle \begin{align*}&\text{Determine whether or not the derivative of the function above at the point }\\&x=-1\\&\text{is positive, negative, or zero.}\end{align*}\)

Possible Answers:

\(\displaystyle \text{The derivative is zero.}\)

\(\displaystyle \text{The derivative is negative.}\)

\(\displaystyle \text{The derivative is positive.}\)

Correct answer:

\(\displaystyle \text{The derivative is positive.}\)

Explanation:

\(\displaystyle \begin{align*}&\text{The derivative of a function at a given point is synonymous}\\&\text{with the slope of a function at a given point. Therefore if}\\&\text{the function increases in value as x increases across a point,}\\&\text{the slope and derivative are positive. Conversely, if the function}\\&\text{decreases in value as x increases across a point, both slope}\\&\text{and derivative are negative. At points where the function is}\\&\text{flat, the slope and derivative are zero.}\\&\text{Studying the slope at }x=-1\text{, the function appears to be increasing.}\\&\text{The derivative is positive.}\end{align*}\)

Example Question #33 : Slope Of A Curve At A Point

Functionplot

\(\displaystyle \begin{align*}&\text{Determine whether or not the derivative of the function above at the point }\\&x=7\\&\text{is positive, negative, or zero.}\end{align*}\)

Possible Answers:

\(\displaystyle \text{The derivative is zero.}\)

\(\displaystyle \text{The derivative is negative.}\)

\(\displaystyle \text{The derivative is positive.}\)

Correct answer:

\(\displaystyle \text{The derivative is positive.}\)

Explanation:

\(\displaystyle \begin{align*}&\text{The derivative of a function at a given point is synonymous}\\&\text{with the slope of a function at a given point. Therefore if}\\&\text{the function increases in value as x increases across a point,}\\&\text{the slope and derivative are positive. Conversely, if the function}\\&\text{decreases in value as x increases across a point, both slope}\\&\text{and derivative are negative. At points where the function is}\\&\text{flat, the slope and derivative are zero.}\\&\text{Studying the slope at }x=7\text{, the function appears to be increasing.}\\&\text{The derivative is positive.}\end{align*}\)

Example Question #34 : Slope Of A Curve At A Point

Functionplot

\(\displaystyle \begin{align*}&\text{Conclude if the derivative of the function above at the point }\\&x=3\\&\text{is positive, negative, or zero.}\end{align*}\)

Possible Answers:

\(\displaystyle \text{The derivative is zero.}\)

\(\displaystyle \text{The derivative is negative.}\)

\(\displaystyle \text{The derivative is positive.}\)

Correct answer:

\(\displaystyle \text{The derivative is positive.}\)

Explanation:

\(\displaystyle \begin{align*}&\text{The derivative of a function at a given point is synonymous}\\&\text{with the slope of a function at a given point. Therefore if}\\&\text{the function increases in value as x increases across a point,}\\&\text{the slope and derivative are positive. Conversely, if the function}\\&\text{decreases in value as x increases across a point, both slope}\\&\text{and derivative are negative. At points where the function is}\\&\text{flat, the slope and derivative are zero.}\\&\text{Studying the slope at }x=3\text{, the function appears to be increasing.}\\&\text{The derivative is positive.}\end{align*}\)

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