SUM OF A.P. 2

Find the sum of the 1st 18 terms of the following arithmetic progression: 5+11+17+…………………

Solution

d=6, A₁ = 5,n=18

S_n = n[2A₁ + (n-1)d]

      2

S₁₈ = 18[2(5) + (16-1)d]

        2

S₁₈ = 9[10 + 15d]

 

S₁₈ = 9[10 + 13(6)]       since d=6

S₁₈ = 9[10 + 78]  

         

S₁₈ = 9 x 88 

          

S₁₈ = 792

Therefore the sum of 1st 16 terms is 792.

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

Find the sum of the 1st 16 terms of the following arithmetic progression: 7+10+13+…………………