SUM OF A.P 1

Find the sum of the 1st 20 terms of the following arithmetic progression: 5+11+17+…………………

Solution

d=6, A₁ = 5,n=20

S_n = n[2A₁ + (n-1)d]

       2

S₂₀ = 20[2(5) + (20-1)d]

         2

S₂₀ = 10[10 + 17d]

 

S₂₀ = 10[10 + 17(6)]       since d=6

S₂₀ = 10[10 + 102]  

         

S₂₀ = 10 x 112 

          

S₂₀ = 1120

Therefore the sum of 1st 20 terms is 1120.

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

Find the sum of the 1st 15 terms of the following arithmetic progression: 6+10+14+…………………