AP Calculus BC : Derivative at a Point

Study concepts, example questions & explanations for AP Calculus BC

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Example Questions

Example Question #21 : Derivative At A Point

Calculate the derivative of \(\displaystyle f(x)=\frac{x^3}{3}-2x^2+x-3\) at the point \(\displaystyle x=3\).

Possible Answers:

\(\displaystyle -3\)

\(\displaystyle -4\)

\(\displaystyle -2\)

\(\displaystyle -1\)

Correct answer:

\(\displaystyle -2\)

Explanation:

There are 2 steps to solving this problem.

First, take the derivative of \(\displaystyle f(x)\)

Then, replace the value of x with the given point and evaluate

For example, if \(\displaystyle x=a\) , then we are looking for the value of \(\displaystyle f'(x=a)\) , or the derivative of \(\displaystyle f(x)\) at \(\displaystyle x=a\).

\(\displaystyle f(x)=\frac{x^3}{3}-2x^2+x-3\)

Calculate \(\displaystyle f'(x)\)

Derivative rules that will be needed here:

  • Derivative of a constant is 0. For example, \(\displaystyle \frac{\mathrm{d} }{\mathrm{d} t} 5 = 0\)
  • Taking a derivative on a term, or using the power rule, can be done by doing the following:\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} t} t^n = n * t^{n-1}\)

\(\displaystyle f'(x)=x^2 - 4x + 1\)

Then, plug in the value of x and evaluate

\(\displaystyle f'(x=3)=3^2-4*3+1=-2\)

Example Question #12 : Derivative Defined As Limit Of Difference Quotient

If \(\displaystyle f(x)=\ln(2x)\), which of the following limits equals \(\displaystyle f'(e+1)\)?

Possible Answers:

\(\displaystyle \lim_{h \to \infty} \frac{\ln(e+1+h)-\ln(e+1)}{e+1}\)

\(\displaystyle \lim_{h \to \infty} \frac{\ln(e+1+h)-\ln(e+1)}{h}\)

\(\displaystyle \lim_{h \to 0} \frac{\ln(1+h)-\ln(h)}{h}\)

\(\displaystyle \lim_{h \to 0} \frac{\ln(e+1+h)-\ln(e+1)}{h}\)

\(\displaystyle \lim_{h \to 0} \frac{\ln(e+1+h)-\ln(e+1)}{e+1}\)

Correct answer:

\(\displaystyle \lim_{h \to 0} \frac{\ln(e+1+h)-\ln(e+1)}{h}\)

Explanation:

The equation for the derivative at a point is given by

\(\displaystyle f'(a)=\lim_{h \to 0}\frac{f(a+h)-f(a)}{h}\).

By substituting \(\displaystyle a = e+1\)\(\displaystyle f = \ln(2x)\), we obtain

\(\displaystyle f'(e+1)=\lim_{h \to 0} \frac{\ln(e+1+h)-\ln(e+1)}{h}.\)

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