Question

If `sum_("r" - 1)^"n" (2"r" + 1) = 440,` then n = _____.

विकल्प

• 20

• 22

• 21

• 19

Answer 1

If `sum_("r" - 1)^"n" (2"r" + 1) = 440,` then n = 20.

Explanation:

We have, `sum_("r" - 1)^"n" (2"r" + 1) = 440`

⇒ 3 + 5 + 7... + (2n + 1) = 440

⇒ `"n"/2[2 xx 3 + ("n" - 1)(2)] = 440`

⇒ n(3 + n - 1) = 440

⇒ n(n + 2) = 440

⇒ n = 20