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If an = \(\cfrac{n(n+1)(2n+1)}6\) then a6 = ……………A) ±1 B) n(n + 1) C) 55 D) 91 |
Answer» Correct option is (D) 91 \(\because\) \(a_n = \frac{n(n+1)(2n+1)}{6}\) \(\therefore\) \(a_6=\frac{6(6+1)(12+1)}{6}\) (By putting n = 6) \(=7\times13=91\) Correct option is D) 91 |
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