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What is the coefficient of x 5 x^{5} in the polynomial expansion ( 2 x x 2 ) 4 (2-x-x^{2})^{4} ?


The answer is -20.

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1 solution

Note that x 5 x^5 can be expressed as either x × x × x × x 2 x \times x \times x \times x^2 or x 2 × x 2 × x x^2 \times x^2 \times x . The first case would have each term with coefficient 1 1 and the second with coefficient 2 -2 , So in essence, the coefficient would be ( 4 3 ) × 1 + ( 4 2 ) × ( 2 1 ) × ( 2 ) = 20 \binom{4}{3} \times 1 + \binom{4}{2}\times \binom{2}{1} \times (-2) =-20

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