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In the expansion of `(x^3-1/(x^2))^n ,n in N`, if the sum of the coefficients of `x^5a n dx^(10)`, then `n`isa. 25 b. 20 c. 15 d. none of theseA. `25`B. `20`C. `15`D. None of these |
Answer» Correct Answer - C `(c )` `((1)/(x^(2))-x^(3))^(n)` `T_(r+1)=(n!)/(r!(n-r)!)(-1)^(n-r)x^(5r-2n)` If `5r-2n=5`, then `5r=2n+5` `r=(2n)/(5)+1` If `5r-2n=10`, then `5r=2n+10` `r=(2n)/(5)+2` Let `n=5k` According to question `(5k!)/((2k+1)!(3k-1)!)-(5k!)/((2k+2)!(3k-2)!)=0` `(1)/(3k-1)-(1)/(2k+2)=0` `k=3`, `n=15` |
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