Question

What is the sum of an 8 term geometric series if the first term is -11 and the last term is 859,375 and the common ratio is -5

Answer 1

Answer:716,144

Step-by-step explanation:

Given

No of the terms in GP $n=8$

First-term $a=-11$

last term $a_n=859,375\quad (ar^{n-1})$

Common ratio $r=-5$

The Sum of a GP is given by

$S_n=\dfrac{a(1-r^n)}{1-r}\quad \quad [r<1]$

Put the values

$\Rightarrow S_n=\dfrac{-11(1-(-5)^8)}{1-(-5)}\\\\\Rightarrow S_n=\dfrac{-11\times -390624}{6}\\\\\Rightarrow S_n=\dfrac{42,96,864}{6}=716,144$