1.

Find the coefficient of `x^(4)` in the expansion of `(1+x+x^(2)+x^(3))^(11)`.

Answer» We have
`(1+x+x^(2) + x^(3))^(11) = {(1+x)+x^(2)(1+x)}^(11)={(1+x)(1+x^(2))}^(11)`
`=(1+x)^(11) xx(1+x^(2))^(11)`
`=(.^(11)C_(0)x^(0)+.^(11)C_(1)x_ +.^(11)C_(2)x^(2)+.^(11)C_(3)x^(3)+.^(11)C_(4)x^(4)+...)`
` xx[.^(11)C_(0)(x^(2))^(0) +.^(11)C_(1)(x^(2)) + .^(11)C_(2)(x^(2))^(2)+...]`
`(1+11x+55x^(2)+165x^(3) + 330x^(4)+...)`
`xx(1+11x^(2)+55x^(4)+...).`
`:. "coefficient of " x^(4) " in the given expansion "`
`=(1 xx55) + ( 55 xx11) +(330 xx1)`
`=(55+605 + 330)=990`.
Hence, the coefficient of `x^(4)` in the given expansion is 990.


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