Question

Obtain the sum of first 56 terms of an A.P whose 18th term and 39th terms are 52 and 148 respectively

Answer 1

a+17d =52
a+51d =148
by solving these eq
we get
d=48/17
a=4
applying formula
sum= n/2(2a+(n-1)d)
where n=56
we get
sum=(2776×28)/17