The coefficient of x13 in the expansion of (1−x)5(1+x+x2+x3)4 is
A
4
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B
−4
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C
0
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D
none of these
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Solution
The correct option is A4 Given, (1−x)5(1+x+x2+x3)4 =(1−x)5((1+x)(1+x2))4 =(1−x)(1−x2)4(1+x2)4 =(1−x)(1−x4)4 =(1−x4)4−x(1−x4)4 Hence the coefficient of x13 will be =−[(−1)34C3] =4