find the coefficient of x9 in the expansion of (1+ 3x + 3x2 +x)15

Dear student
We have,
1+3x+3x2+x315=1+x315=1+x45Suppose x9 occurs in (r+1)th term in the expansion of 1+x45.Now,Tr+1=45CrxrFor this term to conatin x9, we must have r=9.So, coefficient of x9=45C9
Regards
 

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