Question

What is the sum of the first 6 terms of this geometric sequence? –4, –16, –64, –256, …

Answer 1

$a_1=-4;\ a_2=-16;\ a_3=-64;\ a_4=-256;\ ...\\\\r=\dfrac{a_{n+1}}{a_n}\to r=\dfrac{a_2}{a_1}\to r=\dfrac{-16}{-4}=4$

The formula of the sum of a geometric sequence:

$S_n=a_1\cdot\dfrac{1-r^n}{1-r}$

We have:

$a_1=-4;\ n=6;\ r=4$

substitute:

$S_6=-4\cdot\dfrac{1-4^6}{1-4}=-4\cdot\dfrac{1-4096}{-3}=-4\cdot\dfrac{-4095}{-3}=-4\cdot1365=-5460$

Answer: C. -5460