Binomial TheoremHard

Question

Co-efficient of x15 in (1 + x  + x3 + x4)n is :

Options

A. nC15 - 3r nCr
B. nC5r
C. nC3r
D. nC3 - r nC5r

Solution

Co-efficient of x15 in (1 + x + x3 + x4)n
= Co-efficient of x15 in (1 + x3)n (1 + x)n = nC0 nC15 + nC1 nC12 + nC2 nC9 + nC3 nC6 + nC4 nC3 + nC5 nC0

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