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Question
For the following G.P.s, find Sn.
`sqrt(5)`, −5, `5sqrt(5)`, −25, ...
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Solution
Here, a = `sqrt(5)`, r = `-sqrt(5)`
∴ Sn = `("a"(1 - "r"^"n"))/(1 - "r")`
= `(sqrt(5)[1 - (- sqrt(5))^"n"])/(1 - (-sqrt(5))`
= `(sqrt(5))/(1 + sqrt(5))[1 - (- sqrt(5))^"n"]`
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