High School Math : Calculus II — Integrals

Study concepts, example questions & explanations for High School Math

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Understanding Maclaurin Series

Give the \(\displaystyle x^{4}\) term of the Maclaurin series expansion of the function \(\displaystyle f(x) = x \tan^{-1} x\).

Possible Answers:

\(\displaystyle - \frac{1}{12} x^{4 }\)

\(\displaystyle - \frac{1}{4} x^{4 }\)

\(\displaystyle 0\)

\(\displaystyle - \frac{3}{4} x^{4 }\)

\(\displaystyle - \frac{1}{3} x^{4 }\)

Correct answer:

\(\displaystyle - \frac{1}{3} x^{4 }\)

Explanation:

This can most easily be answered by recalling that the Maclaurin series for \(\displaystyle \tan^{-1} x\) is 

\(\displaystyle \tan^{-1} x = x - \frac{1}{3} x^{3 } + \frac{1}{5} x^{5 }...\)

Multiply by \(\displaystyle x\) to get:

 \(\displaystyle x\tan^{-1} x = x\cdot x - x\cdot \frac{1}{3} x^{3 } + x\cdot \frac{1}{5} x^{5 }...\)

\(\displaystyle x \tan^{-1} x=x^{2 } - \frac{1}{3} x^{4 } + \frac{1}{5} x^{6 }...\)

The \(\displaystyle x^{4}\) term is therefore \(\displaystyle - \frac{1}{3} x^{4 }\).

Example Question #2 : Understanding Maclaurin Series

Give the \(\displaystyle x^ {3}\) term of the Maclaurin series of the function \(\displaystyle f(x) = \sinh 3x\) .

Possible Answers:

\(\displaystyle \frac{9}{2} x^{3}\)

\(\displaystyle -9x^{3}\)

\(\displaystyle 9x^{3}\)

\(\displaystyle -\frac{9}{2} x^{3}\)

\(\displaystyle 0\)

Correct answer:

\(\displaystyle \frac{9}{2} x^{3}\)

Explanation:

The \(\displaystyle x^ {3}\) term of a Maclaurin series expansion has coefficient

\(\displaystyle \frac{f ' '' (0)}{3!} = \frac{f ' '' (0)}{6}\).

We can find \(\displaystyle f'''(x)\) by differentiating three times in succession:

\(\displaystyle f(x) = \sinh 3x\)

\(\displaystyle f'(x) = 3 \cosh 3x\)

\(\displaystyle f''(x) = 3\cdot 3\sinh 3x = 9\sinh 3x\)

\(\displaystyle f'''(x) = 9 \cdot 3\cosh 3x = 27 \cosh 3x\)

\(\displaystyle f'''(0) = 27 \cosh \left (3 \cdot 0 \right ) = 27 \cosh 0 = 27\cdot 1 = 27\)

The term we want is therefore

\(\displaystyle \frac{f ' '' (0)}{6} \cdot x^{3} = \frac{27}{6} \cdot x^{3} = \frac{9}{2} x^{3}\)

Example Question #2251 : High School Math

Give the \(\displaystyle x^{2}\) term of the Maclaurin series expansion of the function \(\displaystyle f (x) = 7 ^{x}\).

Possible Answers:

\(\displaystyle \frac{\left ( \ln 7 \right )^{2}}{2} x^{2}\)

\(\displaystyle \left (2 \ln 7 \right )x^{2}\)

\(\displaystyle \frac{7}{2}x^{2}\)

\(\displaystyle \frac{ \ln 7}{2}x^{2}\)

\(\displaystyle \frac{e^{7}}{2}x^{2}\)

Correct answer:

\(\displaystyle \frac{\left ( \ln 7 \right )^{2}}{2} x^{2}\)

Explanation:

The \(\displaystyle x^{2}\) term of a Maclaurin series expansion has coefficient

\(\displaystyle \frac{f ' ' (0)}{2!} = \frac{f ' ' (0)}{2}\).

We can find \(\displaystyle f''(x)\) by differentiating twice in succession:

\(\displaystyle f (x) = 7^{x}\)

\(\displaystyle f ' (x) = \frac{\mathrm{d} }{\mathrm{d} x} \left ( 7^{x} \right )= \ln 7 \cdot 7^{x}\)

 

\(\displaystyle f ' '(x) = \frac{\mathrm{d} }{\mathrm{d} x} \left (\ln 7 \cdot 7^{x} \right )\)

\(\displaystyle f ' '(x)= \ln 7\cdot \frac{\mathrm{d} }{\mathrm{d} x} \left ( 7^{x} \right )\)

\(\displaystyle f ' '(x)= \ln 7\cdot \ln 7\cdot 7^{x}\)

\(\displaystyle f ' '(x)=\left ( \ln 7 \right )^{2} \cdot 7^{x}\)

 

\(\displaystyle f ' '(0)=\left ( \ln 7 \right )^{2} \cdot 7^{0} = \left ( \ln 7 \right )^{2} \cdot 1 = \left ( \ln 7 \right )^{2}\)

The coefficient we want is 

\(\displaystyle \frac{f ' ' (0)}{2} =\frac{\left ( \ln 7 \right )^{2}}{2}\),

so the corresponding term is \(\displaystyle \frac{\left ( \ln 7 \right )^{2}}{2} x^{2}\).

Learning Tools by Varsity Tutors