Question 972330
common ration {{{-2}}}, first term, 8.


You can look for the formula in a textbook or a reference manual:



The sum of the first n terms of geometric sequence, common ratio r, first term {{{A[1]}}} is:
{{{A[1]((1-r^n)/(1-r))}}}


Your series,
{{{8((1-(-2)^9)/(1-(-2)))}}}------simplify.