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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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