The coefficient of x13 in the expansion of (1−x)5(1+x+x2+x3)4 is
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Solution
Let E=(1−x)5(1+x+x2+x3)4 =(1−x)5[(1+x)+x2(1+x)]4 =(1−x)5(1+x)4(1+x2)4 =(1−x)(1−x2)4(1+x2)4 =(1−x)(1−x4)4 =(1−x)[4C0−4C1(x4)+4C2(x4)2−4C3(x4)3+4C4(x4)4] Coefficient of x13=(−1)(−4C3)=4