SEQUENCE AND SERIES A12

The first term of an AP is 18 and the last term is 100.If the arithmetic progression consists of 20 terms, calculate the sum of all the terms.

Solution

A₁ = 18, n=20, A_n = 100

S_n = n(A₁ + A_n)

       2

S₂₀ = 20(18 + 100)

         2

S₂₀ = 10 x 118

S₂₀ = 1180

Hence the sum of all 20 terms is 1180.