Question

the sum of first seven terms of an ap is 49 and sum of 17 terms is 289 , find sum of n terms of ap

Answer 1

7/2(2a+6d)=49
2a+6d=14
a+3d=7 (i)
17/2(2a+16d)=289
2a+16d=28
a+8d=14(ii)
from i &ii we get
d=1.4
a=2.8
sn=n/2{2×2.8+(n-1)d}