Question

Find the number of terms in the G.P. 1, −3, 9, .... −2187.

Answer 1

a = 1

r = `(-3)/1`

r = −3

`a_n = ar^(n - 1)`

` -2187 = 1(-3)^(n - 1)`

`(-3)^(n - 1) = -2187`

`(-3)^(n - 1) = (-3)^7`

n − 1 = 7

n = 7 + 1

n = 8