1.

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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