SUM OF A GP-1A

Find the sum of the 1st nine terms of the geometrical progression 2+6+18+54+…….

solution

G₁ = 2, r=3, n=9

S_n = G₁(rⁿ-1)/r-1

               

S₉ = 2(3⁹-1)/3-1    [data substitution]

             

S₉ = ¹2(3⁹-1)/2₁

              

S₉ = (3⁹-1)

S₉ = 19683 - 1

S₉ = 19682

Hence the sum of the 1st nine terms is 19682.

TRY THIS………………

Find the sum of the 1st seven terms of the geometrical progression 3+9+27+81+…..