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What is the coefficient of x^9 in the expansion of (x+5)^14?(a) 5! * ^14C6(b) ^14C5(c) 54 * ^14C5(d) 34 * ^11C5I got this question during a job interview.Enquiry is from Counting topic in division Counting of Discrete Mathematics

Answer» RIGHT choice is (c) 54 * ^14C5

Explanation: the binomial theorem is (x+y)^a = Σ ^aCi x^a-i y^i. In order to get the coefficient of x^9, we need to have a-i=9. SINCE a=14, i=5. THUS, the ANSWER is ^aC5 * y^4 = 5^4 * ^14C5.


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