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COMEDK2014MathematicsSequences and Series

If the sum of first n natural numbers is 1 78 times the sum their cubes, then the value of n is

Options

  1. A11
  2. B12
  3. C13
  4. D14

Correct answer

B. 12

Step-by-step solution

Since, n= 1 78 n³ array ll & n(n+1) 2 = 1 78 n²(n+1)² 4 & n²+n-156=0 & (n+13)(n-12)=0 & n=12 [n -13] array

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