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Find the coefficient of x^7 in (x+4)^9.(a) 523001(b) 428700(c) 327640(d) 129024This question was addressed to me in an interview.My question is taken from Counting in chapter Counting of Discrete Mathematics

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Right ANSWER is (d) 129024

To EXPLAIN: It is known that (r+1)^TH TERM, in the binomial expansion of (a+b)^n is given by, Tr+1 = ^nCra^n-rb^r. Assuming that x^7 occurs in the (r+1)^th term of the expansion (x+4)^9, we obtain Tr+1 = 129024x^4.



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