Let a, ar, ar², ........ be an infinite G.P. If _ n=0 ^ ar^n = 57 and _ n=0 ^ a^3r^ 3n = 9747, then a + 18r is equal to :

JEE Main 2024 Mathematics question

Let a, ar, ar², ........ be an infinite G.P. If and , then a + 18r is equal to :

Answer: (D)

a/(1 − r) = 57 and a³/(1 − r³) = 9747. Dividing the cube of the first by the second: (1 − r³)/(1 − r)³ = 57³/9747 = 19, i.e. (1 + r + r²)/(1

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Question ID VK-016460