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Questions  

 The coefficient of xn in the expansion of (1+x)(1x)n 

a
(−1)nn
b
(−1)n(1−n)
c
(−1)n−1(n−1)
d
(n−1)

detailed solution

Correct option is B

(1+x)(1−x)n=[2−(1−x)](1−x)n =2(1−x)n−(1−x)n+1 ∴coefficient of  xn in  (1+x)(1−x)n=2(−1)n−n+1Cn(−1)n=(−1)n(2−n−1)=(−1)n(1−n)

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