Question

The sum of first eight terms of an AP is hundred and the sum of its first nineteenth term is 551 .find the AP

Answer 1

S8=100
S19=551
Sn=n/2[2a+(n-1)d]
therefore S8=8/2[2a+(8-1)d]
100=4[2a+7d]
25=2a+7d......(1)
S19=19/2[2a+(19-1)d]
551×2/19=2a+18d
58=2a+18d,......(2)
Subtract (1) and (2)
33=11d
d=3
2a+7×3=25
2a+21=25
2a=4
a=2

therefore the AP is 2,5,8,11,14,17..
Hope it is right