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Question
For a G.P., if S5 = 1023, r = 4, find a.
Solution
r = 4, S5 = 1023
Sn = `"a"(("r"^"n" - 1)/("r" - 1))`, for r > 1
∴ S5 = `"a"((4^5 - 1)/(4 - 1))`
∴ 1023 = `"a"((1024 - 1)/3)`
∴ 1023 = `"a"/3(1023)`
∴ a = 3.
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