GEOMETRIC PROGRESSION 1

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

solution

G₁ = 2, r=3, n=11

    

S_n = G₁(rⁿ-1)/r-1

               

S₁₁ = 2(3¹¹-1)/3-1    [data substitution]

             

S₁₁ = ¹2(3¹¹-1)/2₁

              

S₁₁ = (3¹¹-1)

S₁₁ = 177147 - 1

S₁₁ = 177146

Hence the sum of the 1st 11 terms is 177146.

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Find the sum of the 1st 8 terms of the geometrical progression 7+14+28+…..