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Question

Coefficient of x11 in the expansion of (1+x2)4(1+x3)7(1+x4)12 is

A
1051
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B
1106
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C
1113
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D
1120
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Solution

The correct option is C 1113
2a+3b+4c=11
Possible combination of (a,b,c)
(0,1,2),(1,3,0),(2,1,1),(4,1,0)

Hence, the coefficient of x11 in (1+x2)4(1+x3)7(1+x4)12 is :

( 4C0× 7C1× 12C2)+( 4C1× 7C3× 12C0)+( 4C2× 7C1× 12C1)+( 4C4× 7C1× 12C0)=462+140+504+7=1113

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