The sum of first four terms is a G. P. is 30 and that of the last four terms is 960. if its first term is 2 find the common ratio .

If the sum of first four terms in a G.P. is 30 and that of the last four terms is 960, then the sum of the first 8 terms is 990

We also know that a = 2

sum of first 4 terms

= 2(r^4 - 1)(r-1) = 30

r^4 - 1 = 15r - 15

r^4 - 15r + 14 = 0

r = 1 works for the equation , but r = 1 would make all the terms of the

GP the same, so clearly not a solution

r^4 - 15r + 14 = 0 , a bit of trial and error

(r-1)(r - 2)(r^2 + 3r + 7) = 0

so r = 2 or r = not real

check: sum(4) = 2(2^4 - 1)/(2-1) = 30 , as needed

so r = 2 , BUT it does not work for sum(8)

sum(8) = 2(2^8 - 1)(2-1) = 510 , not 990 as necessary according to your second part of the question.

check your question.

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