Question

if in an Ap the sum of first 10 terms =100 and 15 terms =225,then find a, d

Answer 1

s10=n/2(2a+(n-1)d). s15=n/2(2a+14d)
100=5(2a+9d). 225=15/2(2a+14d)
20=2a+9d. 30=2a+14d
on solving
5d=10
d=2
a=(20-18)/2
=1