Question

the sum of first three terms of a gp is 15 and the sum of next 3 terms is 120.find the sum of first n terms.

​

Answer 1

Answer:

Step-by-step explanation:

terms are a,ar,ar^2,ar^3,ar^4,ar^5

a+ar+ar^2=15

a(1+r+r^2)=15

ar^3+ar^4+ar^5=120

ar^3(1+r+r^2)=120

ar^3/a=120/15

r^3=8

r=2

a(1+2+4)=15

a=15/7

sum of first n terms=a(r^n-1)/(r-1)

=(15/7)(2^n-1)