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Question

Coefficient of x4 in (1+x+x2+x3)11 is

A
55
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B
605
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C
990
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D
1120
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Solution

The correct option is C 990
(1+x+x2+x3)11
=(1+x2)11(1+x)11
=(1+11x2+11C2x4+...x22)(1+11x+11C2x2+...x11)
Hence coefficient of x4 will be
1×11C4+11×11C2+11C2×1
=11C4+12×11C2
=330+660
=990

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