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