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GATE 2021 AG – Question 4

General Aptitude · Quantitative Aptitude: Algebra, Geometry and Mensuration · 1 mark · Multiple choice

Which number is exactly divisible by $11^{13}+1$?

  1. $11^{26}+1$
  2. $11^{33}+1$
  3. $11^{39}-1$
  4. $11^{52}-1$

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Correct answer: (D) $11^{52}-1$

Explanation

Put a=11¹³. Modulo a+1, a≡−1, so $a^4-1\equiv1-1=0$. Therefore $11^{52}-1$ is divisible, giving **D**. A gives remainder2 and C gives−2;33 is not an odd multiple of 13, so the usual odd-power factorization does not apply to B.

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