Question 1202481
.



        The formula and the final answer/value in the post by @mananth (to which he refers) are incorrect.

        I came to bring a correct solution/answer.



<pre>
{{{S[n]}}} = {{{a*((1-r^n)/(1-r))}}}


a = 8

r = -1/2

{{{S[17]}}} = {{{8*(1-(-0.5)^17))/(1-(-0.5))}}}

{{{S[17]}}} = 5.3333740234375  (exact value).
</pre>

Solved (correctly).