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The coefficient of `x^(7)` in the expansion of `(1-x-x^(2) + x^(3))^(6)` is :A. 144B. -132C. -144D. 132 |
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Answer» Correct Answer - c We have `(1 - x - x^(2) + x^(3))^(6) = {(1 - x)( 1 - x^(2))}^(7) = (1 - x )^(7) = (1 - x)^(7) (1 - x^(2))^(7)` = `(""^(6)C_(0) - ""^(6)C_(1) x + ""^(6)C_(2)x^(2) - ""^(6)C_(3)x^(3) +... + ""^(6)C_(6)x^(6))` `(""^(6)C_(0) - ""^(6)C_(1) x^(2) + ""^(6)C_(2)x^(4) - ""^(6)C_(3)x^(6) +... )` `therefore` Coefficient of `x^(7)` in `(1 - x - x^(2) + x^(3))^(6)` ` = - ""^(6)C_(1)xx (-""^(6)C_(3)) - ""^(6)C_(3)xx ""^(6)C_(3)xx ""^(6)C_(2) - ""^(6)C_(5) xx - (""^(6)C_(1))` ` 120 - 300 + 36 = -144` . |
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