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Questions  

The coefficient of x4 in the expansion of 1+x+x2+x3n is

a
nC4
b
nC4+nC2
c
nC4+nC2+nC2
d
nC4+nC2+nC1⋅nC2

detailed solution

Correct option is D

1+x+x2+x3n=(1+x)n1+x2n =1+nC1x+nC2x2+…+nCnxn1+nC1x2+nC2x4+…+nCnx2n .Therefore, the coefficient of x4=nC2+nC2nC1+nC4=nC4+nC2+nC1nC2

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