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In a GP the product of the first term and the 7th term is equal to the 4th term. Given that the sum of the 1st and 4th is 9. find the sum of the first 6 terms

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a*ar^6 = ar^3
so ar^3 = 1

a + ar^3 = 9
so now just solve for a and r

and as usual,
S6 = a(1-r^6)/(1-r)

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