For what values of x, the numbers (x + 9), (x - 6) and 4 are in G.P?

The 4th, 6th and last term of a geometric progression are 10, 40 and 640 respectively. If the common ratio is positive, find the first term, common ratio and the number of terms of the series.

Find the sum of first 8 terms of the G.P. 1, 3, 9, 27, 81, …..