Question

Find the sum of the first 5 terms of the geometric series -2, -6, -18, …

Answer 1

a₁ = -2
a₂ = -6


`r= (a_2)/(a_1)= (-6)/(-2) =3 \\ \\ \\ S_5= \cfrac{a_1(1-r^5)}{1-r}= \cfrac{-2(1-3^5)}{1-3}=\cfrac{-2(1-243)}{-2}=1-243=-242`

Answer: -242