Question

The sum of first 9 terms of an AP is 81 and the sum of first 15 terms is 225. Find the sum of first n terms

Answer 1

Answer:

then a+4d=9

a+7d=15

3d=6

d=2

a=1

ok

follow

me

gys

Answer 2

Answer:

HERE IS UR ANS:-

Sum of 1st 9 terms = 81

==> a + 8d = 81

==> a = 81 - 8d ----->(1)

Sum of 1st 15 terms = 225

==> a + 14d = 225

==> 81 - 8d + 14d = 225 [From (1)]

==> 6d = 144

==> d = 24

==> a = -111

Sum of n terms ==> (n/2)(2a + (n - 1)d)

==>

$12 n {}^{2} - 123n$

Hope it helps you.

Pls Mark The Brainliest.