# Thread: How to estimate summations

1. ## How to estimate summations

i.e.

$\displaystyle \sum_{n = 0}^{\infty}\frac{1}{2^{n}} = \frac{1}{2^{0}} + \frac{1}{2^{1}} + \frac{1}{2^{2}} + \frac{1}{2^{3}} + \frac{1}{2^{4}} + \frac{1}{2^{5}} + \frac{1}{2^{6}} + \cdots = ~1.99138889...$

Is there a way you can know this solution is 2 without having to perform all of the calculations I did to find which number the sums are approaching? And is there a general method for questions like these to find the solution without having to perform a lot of calculations?

2. ## Re: How to estimate summations

Originally Posted by daigo
i.e.

$\displaystyle \sum_{n = 0}^{\infty}\frac{1}{2^{n}} = \frac{1}{2^{0}} + \frac{1}{2^{1}} + \frac{1}{2^{2}} + \frac{1}{2^{3}} + \frac{1}{2^{4}} + \frac{1}{2^{5}} + \frac{1}{2^{6}} + \cdots = ~1.99138889...$

Is there a way you can know this solution is 2 without having to perform all of the calculations I did to find which number the sums are approaching? And is there a general method for questions like these to find the solution without having to perform a lot of calculations?
Yes, this is an infinite geometric series with \displaystyle \displaystyle \begin{align*} a = 1, r = \frac{1}{2} \end{align*}, so the sum is \displaystyle \displaystyle \begin{align*} \frac{a}{1 - r} = \frac{1}{1 - \frac{1}{2}} = \frac{1}{\frac{1}{2}} = 2 \end{align*}.

3. ## Re: How to estimate summations

The sum of a geometric series is given by

$\displaystyle \sum_{i=0}^{\infty} r^i = \frac{1}{1-r}$, given $\displaystyle |r| < 1$. In this case, $\displaystyle r = \frac{1}{2}$.