Question

For a G.P., a = 2 and r = 3; then find the sum of first four terms.

Answer 1

a = 2, r = 3, n = 4

Sn = \(\frac{a[r^n−1]}{r−1}\)

Putting n = 4; a = 2; r = 3 in the formula,

Sn = \(\frac{2[(3)^4−1]}{3−1}=\frac{2[81−1]}{2}\) = 80

Hence, the sum of first four terms of a G.P. Is 80.