Question

Find the sum of first n terms of an AP whose (n)th term is (5n-1). Hence, find the sum of first 20 terms.

Answer 1

an=5n-1

a1=5x1-1

a=4

Sn=n/2[a+an]

Sn=n/2[4+5n-1]

Sn=n/2x5n+3

Sn=5n²+3n/2

S20=5(20)²+3(20)/2

S20=1030