1.

If `sum _(r=1)^(n) (2r +1) =440` , then n = ……A. 20B. 22C. 21D. 19

Answer» Correct Answer - A
We have `sum_(r=1)^(n) (2r +l) = 440`
`rArr " "3+5 + 7 .....+(2r +l) =440`
`rArr" "(n)/(2) [2xx 3 +(n-1)(2)] = 440`
`rArr" "n(3+n-1) = 440`
`rArr " "n(n+2) = 440`
`rArr " "n = 20`


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