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Which term of G.P. 25, 125, 625, …………. is 390625?(a) 5(b) 6(c) 7(d) 8This question was addressed to me in an internship interview.My enquiry is from Geometric Progression(G.P.) topic in chapter Sequences and Series of Mathematics – Class 11

Answer»

Right option is (c) 7

For EXPLANATION: In the given G.P., a=25 and r = 125/25 = 5.

Given, an = 390625 => ar^n-1 = 390625

=>25*5^n-1 = 390625

=> 5^n-1 = 390625/25 = 15625 = 5^6

=> n-1 = 6 => n=7.



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