1.

The coefficient of `x^(7)` in the expansion of `(1-x-x^(2) + x^(3))^(6)` is :A. 144B. -132C. -144D. 132

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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