For the following G.P.*, find Sn

$sqrt(5)$, 5, $5sqrt(5)$, 25 ...

Here, a = $sqrt(5)$, r = $-sqrt(5)$

Sn = $("a"(1 - "r"^"n"))/(1 - "r")$

= $(sqrt(5)[1 - (- sqrt(5))^"n"])/(1 - (-sqrt(5))$

= $(sqrt(5))/(1 + sqrt(5))[1 - (- sqrt(5))^"n"]$