Multinomial

Algebra Level 2

Find coefficient of x 6 x^6 in ( 1 x + 2 x 2 ) 10 . \big(1-x+2x^2\big)^{10}.

0 210 960 2520 5040 8730

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1 solution

Andy Hayes
Aug 4, 2015

An x 6 x^6 term can be formed from the following products in the expansion:

( 2 x 2 ) 3 ( 1 ) 7 = 8 x 6 (2x^2)^3(1)^7=8x^6 . This occurs ( 10 3 ) = 120 {10\choose3}=120 times in the expansion.

( 2 x 2 ) 2 ( x ) 2 ( 1 ) 6 = 4 x 6 (2x^2)^2(-x)^2(1)^6=4x^6 . This occurs ( 10 2 ) ( 8 2 ) = 1260 {10\choose2}{8\choose2}=1260 times in the expansion.

( 2 x 2 ) 1 ( x ) 4 ( 1 ) 5 = 2 x 6 (2x^2)^1(-x)^4(1)^5=2x^6 . This occurs ( 10 1 ) ( 9 4 ) = 1260 {10\choose1}{9\choose4}=1260 times in the expansion.

( x ) 6 ( 1 ) 4 = x 6 (-x)^6(1)^4=x^6 . This occurs ( 10 6 ) = 210 {10\choose6}=210 times in the expansion.

Thus, the coefficient of the x 6 x^6 term is 8 × 120 + 4 × 1260 + 2 × 1260 + 210 = 8730 8\times120+4\times1260+2\times1260+210=\boxed{8730} .

clear explanation, is there an alternative solution

Renato Mame - 7 months, 1 week ago

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