Multiple choice general knowledge math & puzzles

let, Sn=3+8+15+24+35+....to n terms and S'n=6+11+18+27+... to n terms,then value of S20-S'20 equals

  1. -45

  2. -60

  3. -20

  4. none of these
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B Correct answer
Explanation

Sn has terms 3, 8, 15, 24, 35... where the k-th term is k²+2k. S20 = sum from k=1 to 20 of (k²+2k) = 2870 + 420 = 3290. S'n has terms 6, 11, 18, 27... where the k-th term is k²+2k+3. S'20 = sum from k=1 to 20 of (k²+2k+3) = 2870 + 420 + 60 = 3350. Therefore S20 - S'20 = 3290 - 3350 = -60.

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