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← JEE Main 2022 · 27 Jun · Shift 1
MathematicsInteger+4 / -1Question 27 of 90

JEE Main 2022 · 27 Jun · Shift 1, Mathematics question 27

If the sum of the first ten terms of the series
15+265+3325+41025+52501+…\frac15+\frac2{65}+\frac3{325}+\frac4{1025}+\frac5{2501}+\dots
is mn\dfrac mn, where m and n are co-prime numbers, then m+nm+n is equal to ______.

Answer and worked solution

Official answer: 276

General term. n4n4+1\dfrac n{4n^4+1}, with 4n4+1=(2n2−2n+1)(2n2+2n+1)4n^4+1=(2n^2-2n+1)(2n^2+2n+1), so the term is 14[12n2−2n+1−12n2+2n+1]\frac14\left[\frac1{2n^2-2n+1}-\frac1{2n^2+2n+1}\right].

Telescope to n=10n=10: 14(1−1221)=55221\frac14\left(1-\frac1{221}\right)=\frac{55}{221}.

So m+n=276m+n=276.

Answer: 276