ARITHMETIC PROGRESSION 1

The first term of an AP is 11 and the last term is 57.If the arithmetic progression consists of 20 terms, calculate the sum of all the terms. 

Solution

A₁ = 11, n=20, A_n = 57

S_n = n(A₁ + A_n)

     2

S₂₀ = 20(11 + 57)

       2

S₂₀ = 10 x 68

S₂₀ = 680

Hence the sum of all 23 terms is 680.

TRY THIS..............

The first term of an AP is 11 and the last term is 33.If the arithmetic progression consists of 40 terms, calculate the sum of all the terms.