Q.

The coefficient of x11 in the expansion of (1+x2)4(1+x3)7(1+x4)12  is equal to ____

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answer is 1113.

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Detailed Solution

(1+4c1x2+4c2x4+4c3x6+4c4x8)(1+7c1x3+7c2x6+7c3x9+7c4x12)×   (1+12c1x4+12c2x8+...)

coefficient of x11=(7c1×12c2)+(4c1×7c3)+(7c1×12c1×4c2)+(4c4×7c1)

=462+140+504+7=1113

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