Question

Question #ddfbf

Answer 1

Sum of the first `20` terms is `-3486784400`

Explanation:

In the geometric sequence `4 , -12,36 ,-108 ...`, first term is

`a_1=4` , common ratio is `(-12)/4=36/-12=-3` , number of terms

is `n` . Sum `s_n= a_1 * (r^n-1)/(r-1) = 4 * ((-3)^20-1)/(-3-1)` or

`s_n= cancel4 * ((-3)^20-1)/-cancel4= - ((-3)^20-1= -3486784400`

Sum of the first `20` terms is `-3486784400` [Ans]