Sum of Nth Term

Find the sum the first 25th terms of following series:

2,5,8,11,14,⋯

Formula to find nth term:

Formula to find sum of n terms:

Solution

# First find the 40 th term:
a40 = a1+(n−1)d
= 2+39(3)
= 119

# Then find the sum:
Sn = n(a1+an)/2
= 40(2+119)/ 2
= 2420