Coefficient Trouble Again ?

What is the coefficient of x 11 x^{11} in the expansion of ( 1 + x 2 ) 4 ( 1 + x 3 ) 7 ( 1 + x 4 ) 12 (1+x^2)^4(1+x^3)^7(1+x^4)^{12}


The answer is 1113.

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1 solution

Vishnu Bhagyanath
May 22, 2015

The following 4 possibilities can occur :

( x 2 ) 0 × ( x 3 ) 1 × ( x 4 ) 2 (x^2)^0 \times (x^3)^1 \times (x^4)^2

( x 2 ) 1 × ( x 3 ) 3 × ( x 4 ) 0 (x^2)^1 \times (x^3)^3 \times (x^4)^0

( x 2 ) 2 × ( x 3 ) 1 × ( x 4 ) 1 (x^2)^2 \times (x^3)^1 \times (x^4)^1

( x 2 ) 4 × ( x 3 ) 1 × ( x 4 ) 0 (x^2)^4 \times (x^3)^1 \times (x^4)^0

Since there are 4 4 terms of ( 1 + x 2 ) (1+x^2) , 7 7 termsof ( 1 + x 3 ) (1+x^3) , 12 12 terms of ( 1 + x 4 ) (1+x^4) . sum of coefficients would be - ( 4 0 ) ( 7 1 ) ( 12 2 ) + ( 4 1 ) ( 7 3 ) ( 12 0 ) + ( 4 2 ) ( 7 1 ) ( 12 1 ) + ( 4 4 ) ( 7 1 ) ( 12 0 ) \binom{4}{0} \binom{7}{1} \binom{12}{2} + \binom{4}{1} \binom{7}{3} \binom{12}{0} + \binom{4}{2} \binom{7}{1} \binom{12}{1} + \binom{4}{4} \binom{7}{1} \binom{12}{0} 462 + 140 + 504 + 7 = 1113 462 + 140 + 504 + 7 = \boxed{1113}

Have another way but i am new to brilliant so iI cant upload my method like how to express my ans :|

Aditya Garg - 6 years ago

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