Very efficient coefficient

What is the value of the coefficient x 2 y 4 z 8 x^2 y^4 z^8 in the expression ( x + y + z ) 14 (x+y+z)^{14} ?


The answer is 45045.

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1 solution

Sabhrant Sachan
May 7, 2016

From a Property of Binomial Thm. Coefficient of a term in ( a + b + c + d + . . . ) n is : n ! n 1 ! × n 2 ! × n 3 ! × n 4 ! × . . . . × a n 1 × b n 2 × c n 3 × d n 4 × . . . . Where n 1 + n 2 + n 3 + n 4 + = n and 0 n 1 , n 2 , n 3 , n 4 , . . . n In this case , we got ( x + y + z ) 14 And we need to find coefficient of x 2 y 4 z 8 coefficient of x 2 y 4 z 8 = 14 ! 2 ! × 4 ! × 8 ! 14 × 13 × 12 × 11 × 10 × 9 2 × 4 × 3 × 2 Ans = 45045 \text {From a Property of Binomial Thm.} \\ \text{Coefficient of a term in } (a+b+c+d+...)^n \text { is :} \\ \dfrac{n!}{{n_1!}\times{n_2!}\times{n_3!}\times{n_4!}\times{....}}\times{a^{n_1}}\times{b^{n_2}}\times{c^{n_3}}\times{d^{n_4}\times{....}} \\ \text{Where } n_1+n_2+n_3+n_4+\cdots=n \text{ and } 0\le{n_1,n_2,n_3,n_4,...}\le{n}\\ \text{In this case , we got } (x+y+z)^{14} \text{ And we need to find coefficient of } x^2y^4z^8 \\ \text{coefficient of } x^2y^4z^8=\dfrac{14!}{2!\times{4!}\times{8!}} \\ \implies \dfrac{14\times{13}\times{12}\times{11}\times{10}\times{9}}{2\times{4}\times{3}\times{2}}\\ \text{Ans }=\boxed{45045}

Very nicely explained! .The property that you have described here is called Multinomial Theorem . :)

Abhay Tiwari - 5 years, 1 month ago

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Thanks :)

Sabhrant Sachan - 5 years, 1 month ago

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My pleasure ;)

Abhay Tiwari - 5 years, 1 month ago

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