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December 22, 2014

December 22, 2014

Posted by **kingsley** on Wednesday, October 5, 2011 at 6:15am.

- maths -
**Reiny**, Wednesday, October 5, 2011 at 8:39amt(5) = ar^4 = 567

t(3) = ar^2 = 63

divide first equation by the second

ar^4/(ar^2) = 567/63

r^2 = 9

r = ± 3

if r = ±3,

ar^2= 63

a = 7

Since you only want 7 terms, it would probably be faster to just find them and add them up

that is,

7 + 21 + 63 + 189 + 567 + 1701 + 5103 = 7651

or 7 - 21 + 63 - 189 + 567 - 1701 - 5103 = ......

or by formula

if a=7, r= 3

sum(7) = 7(3^7 - 1)/(3-1) = 7651

or if x=-3

sum(7) = 7((-3)^7 - 1)/(-3-1) = .....

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