Question

The first term of an A.P. is 5, the last term is 45 and the sum is 400. Find the number of terms and the common difference.

Answer 1

Given that, a=5,l=45 and Sn=400

Sn=n/2 (a+l)
400=n/2 (5+45)
400=n/2 (50)
n=16

l=a+(n-1)d
45=5+(16-1)d
40=15d
d=40/15
=8/3 (answer)