Question

20. The sum of the first six terms of a G.P. is 7 times the sum of the first two
terms. Find the common ratio,
​

Answer 1

Answer:

Let S

n

be sum first n terms of G.P. and r be common ratio

Formula for sum of n terms of G.P. is given by

S

n

=

r−1

a(r

n

−1)

Given :

S

6

=9×S

3

S

6

=

r−1

a(r

6

−1)

=9×

r−1

a(r

3

−1)

⇒r

6

−1=9(r

3

−1)

⇒(r

3

−1)(r

3

+1)=9(r

3

−1)

⇒r

3

=−1+9=8

⇒r=2

in this way you can solve your problem