SUM OF AP 2

The first term of an AP is 21 and the last term is 97. If the arithmetic progression consists of 20 terms, calculate the sum of all the terms. 

Solution

A₁ = 21, n=20, A_n = 97

S_n = n(A₁ + A_n)

       2

S₂₀ = 20(21 + 97)

         2

S₂₀ = 10 x 118

S₂₀ = 1180

Hence the sum of all 20 terms is 1180.

TRYTHIS………………….

The sum of first 20 terms of an AP is 1380, and the last term is 97. Find the 1^st term.