High School Math : Finding Sums of Infinite Series

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #2061 : High School Math

Find the value for \(\displaystyle \sum_{i=0}^{\infty} (\frac{1}{2})^i\)

Possible Answers:

\(\displaystyle 1\)

\(\displaystyle 2\)

\(\displaystyle 1.25\)

\(\displaystyle 1.5\)

\(\displaystyle 0\)

Correct answer:

\(\displaystyle 2\)

Explanation:

To best understand, let's write out the series. So

\(\displaystyle \sum_{i=0}^{\infty} (\frac{1}{2})^i = 1+1/2+(1/2)^2 + (1/2)^3+...+(1/2)^\infty\)

We can see this is an infinite geometric series with each successive term being multiplied by \(\displaystyle \frac{1}{2}\).

A definition you may wish to remember is

\(\displaystyle 1+r+r^2+r^3+... +r^\infty= 1/(1-r)\) where \(\displaystyle r\) stands for the common ratio between the numbers, which in this case is \(\displaystyle \frac{1}{2}\) or \(\displaystyle .5\). So we get

\(\displaystyle 1/(1-r) = 1/(1-.5) = 1/.5 = 2\) 

Example Question #31 : Sequences And Series

Evaluate:

\(\displaystyle 1 + \frac{1}{7} + \frac{1}{49} + \frac{1}{343} +...\)

Possible Answers:

The series does not converge.

\(\displaystyle \frac{4}{3}\)

\(\displaystyle \frac{6}{5}\)

\(\displaystyle \frac{9}{8}\)

\(\displaystyle \frac{7}{6}\)

Correct answer:

\(\displaystyle \frac{7}{6}\)

Explanation:

This is a geometric series whose first term is  \(\displaystyle a_{0} = 1\) and whose common ratio is \(\displaystyle r = \frac{1}{7}\). The sum of this series is:

\(\displaystyle \frac{a_{0}}{1-r} = \frac{1}{1- \frac{1}{7} } = \frac{1}{\frac{7}{7}- \frac{1}{7}}= \frac{1}{\frac{6}{7}} =\frac{7}{6}\)

Example Question #2 : Finding Sums Of Infinite Series

Evaluate:

\(\displaystyle 1 - \frac{1}{6} + \frac{1}{36} - \frac{1}{216} +...\)

Possible Answers:

\(\displaystyle \frac{7}{8}\)

\(\displaystyle \frac{2}{3}\)

\(\displaystyle \frac{6}{7}\)

The series does not converge.

\(\displaystyle \frac{4}{5}\)

Correct answer:

\(\displaystyle \frac{6}{7}\)

Explanation:

This is a geometric series whose first term is  \(\displaystyle a_{0} = 1\) and whose common ratio is \(\displaystyle r = - \frac{1}{6}\). The sum of this series is:

\(\displaystyle \frac{a_{0}}{1-r} = \frac{1}{1- \left (- \frac{1}{6} \right )} = \frac{1}{\frac{6}{6}+ \frac{1}{6}}= \frac{1}{\frac{7}{6}} =\frac{6}{7}\)

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