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JEE Main 20265 AprilEvening Shift
Exam
JEE Main
Year
2026
Date
5 April
Shift
Evening Shift
Subject
Mathematics
Chapter
Sequences and Series

JEE Main 2026 · 5 April · Evening Shift · Mathematics · Sequences and Series

If the sum of the first 10 terms of the series (1)/(1 + 1^4 × 4) +...

If the sum of the first 1010 terms of the series 11+14×4+21+24×4+31+34×4+41+44×4+…\dfrac{1}{1 + 1^4 \times 4} + \dfrac{2}{1 + 2^4 \times 4} + \dfrac{3}{1 + 3^4 \times 4} + \dfrac{4}{1 + 4^4 \times 4} + \ldots is mn\dfrac{m}{n}, gcd⁡(m,n)=1\gcd(m, n) = 1, then m+nm + n is equal to :

Options

  1. A256256
  2. B264264
  3. C276276
  4. D284284

Correct answer

C. 276276

Step-by-step solution

The general term of the series is Tr=r1+4r4T_r = \dfrac{r}{1 + 4r^4}.

Factorizing the denominator:

1+4r4=(1+2r2)2−4r2=(2r2−2r+1)(2r2+2r+1)1 + 4r^4 = (1 + 2r^2)^2 - 4r^2 = (2r^2 - 2r + 1)(2r^2 + 2r + 1)

Thus, Tr=r(2r2−2r+1)(2r2+2r+1)T_r = \dfrac{r}{(2r^2 - 2r + 1)(2r^2 + 2r + 1)}

Multiplying and dividing by 44:

Tr=14[4r(2r2−2r+1)(2r2+2r+1)]T_r = \dfrac{1}{4} \left[ \dfrac{4r}{(2r^2 - 2r + 1)(2r^2 + 2r + 1)} \right]

Tr=14[12r2−2r+1−12r2+2r+1]T_r = \dfrac{1}{4} \left[ \dfrac{1}{2r^2 - 2r + 1} - \dfrac{1}{2r^2 + 2r + 1} \right]

The sum of the first 1010 terms is:

S10=∑r=110Tr=14∑r=110(12r2−2r+1−12(r+1)2−2(r+1)+1)S_{10} = \sum_{r=1}^{10} T_r = \dfrac{1}{4} \sum_{r=1}^{10} \left( \dfrac{1}{2r^2 - 2r + 1} - \dfrac{1}{2(r+1)^2 - 2(r+1) + 1} \right)

This is a telescoping series, so all intermediate terms cancel out:

S10=14[(11−15)+(15−113)+…+(12(10)2−2(10)+1−12(10)2+2(10)+1)]S_{10} = \dfrac{1}{4} \left[ \left( \dfrac{1}{1} - \dfrac{1}{5} \right) + \left( \dfrac{1}{5} - \dfrac{1}{13} \right) + \ldots + \left( \dfrac{1}{2(10)^2 - 2(10) + 1} - \dfrac{1}{2(10)^2 + 2(10) + 1} \right) \right]

S10=14[1−1221]=14(220221)=55221S_{10} = \dfrac{1}{4} \left[ 1 - \dfrac{1}{221} \right] = \dfrac{1}{4} \left( \dfrac{220}{221} \right) = \dfrac{55}{221}

Given S10=mnS_{10} = \dfrac{m}{n} and gcd⁡(m,n)=1\gcd(m, n) = 1.

Here, m=55m = 55 and n=221n = 221. Since 55=5×1155 = 5 \times 11 and 221=13×17221 = 13 \times 17, gcd⁡(55,221)=1\gcd(55, 221) = 1.

Therefore, m+n=55+221=276m + n = 55 + 221 = 276.

Answer: 276276
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