Question

The n^th term of a G.P. is 3.(–2)ⁿ. Find the sum of its 7 terms.

Answer 1

N = 1, a₁ = 3.(–2)¹

= –6

N = 2, a₂ = 3.(–2)²

= 3.4

= 12

r = `12/(-6)`

= –2

`S_n = (a(r^n - 1))/(r - 1)`

`S_7 = (-6.((-2)^7 - 1))/(- 2 - 1)`

`S_7 = (-6.(-128 - 1))/(- 3)`

S₇ = 2.(−129)

S₇ = −258