SUM OF AP 1

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) + (18-1)d]

         2

S₁₈ = 9[10 + 17d]

 

S₁₈ = 9[10 + 17(6)]       since d=6

S₁₈ = 9[10 + 102]  

         

S₁₈ = 9 x 112 

          

S₁₈ = 1008

Therefore the sum of 1st 18 terms is 1008.

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

Find the sum of the 1st 12 terms of the following arithmetic progression: 6+10+14+…………………