Question

If for an A.P., a = 10, t15 = 20, then find S15

Answer 1

Answer:

let common difference is d

so 15th term=20=10+(15-1).d

=> 20=10+14d

=> 14d=20-10

=> 14d=10

=> d=10/14=5/7

S15=a/2{2a+(15-1).d}

=10/2{2×10+14.5/7}

=5{20+10}

=5×30

=150