Question

in a gp, the sum of n terms is 225 the last term is 128 and common ratio is 2, find n​

Answer 1

Answer:

Given T

n

=ar

n−1

=128,r=2

S

n

=

r−1

a(r

n

−1)

=255 or

r−1

ar

n−1

.r−a

=255 or

2−1

128.2−a

=255

∴256−255=a or a=1.

Now ar

n−1

=128

∴1.(2)

n−1

=2

7

∴n−1=7orn=8.