Question

the sum of the first six terms of a G.P is 9 times the sum of the first three terms . what is the first term ?​

Answer 1

Without any more information, the first term can be any number.

Let a be the first term in the geometric progression, and let r be the common ratio between consecutive terms. Then

a + ar + ar ² + ar ³ + ar ⁴ + ar ⁵ = 9 (a + ar + ar ²)

ar ³ + ar ⁴ + ar ⁵ = 8 (a + ar + ar ²)

r ³ + r ⁴ + r ⁵ = 8 (1 + r + r ²)

r ³ (1 + r + r ²) = 8 (1 + r + r ²)

r ³ = 8

r = 2

Now with this ratio, the sum of the first six terms is

a (1 + r + r ² + r ³ + r ⁴ + r ⁵) = a (1 - r ⁶)/(1 - r) = 63a

while the sum of the first three terms is

a (1 + r + r ²) = a (1 - r ³)/(1 - r) = 7a

and of course 63a = 9 • 7a, and this is true for any a.