Multiple choice general knowledge math & puzzles

The sum of first “25” numbers in the following series is having one of it’s factors as: 1 4 9 16 25 36 …………………………….

  1. 4

  2. 10

  3. 19

  4. 17

  5. 21

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D Correct answer
Explanation

The series is of squares: 1, 4, 9... n^2. The sum of the first n squares is [n(n+1)(2n+1)]/6. For n=25: (25 * 26 * 51) / 6 = 25 * 13 * 17 = 5525. Since 17 is a prime factor in the formula (2n+1 = 51, which is 3*17), 17 is a factor.