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Question

The coefficient of x4 in the expansion of (1+x+x2+x3)11 is:

A
991
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B
990
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C
989
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D
None of these
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Solution

The correct option is B 990
We have,
(1+x+x2+x3)11=(1+x+x2(1+x))11(1+x)11(1+x2)11(coefficientof1in(1+x)11)(coefficientofx4in(1+x2)11)(coefficientofx2in(1+x)11)+(coefficientofx2in(1+x2)11)+(coefficientofx4in(1+x)11)(coefficientof1in(1+x2)11)1×11C2+11C2×11C1+11C4×111!a!2!+11!a!2!×11+11!7!4!55+605+380=990

Hence, this is the answer.

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