The coefficient of x7 in the expansion of (1−x4)(1+x)9 is
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a
9C7
b
9C3
c
9C7−9C3
d
None of these
answer is C.
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Detailed Solution
Given expansion(1−x4)(1+x)9 In these expansion (1+x)9 be the in the form (a+x)n= nC0 an+ nC1 an−1 x+...+ nCn xn then we get coefficient of x7 (1−x4)(1+x)9=(1−x4)× (9C0+9C1 x+9C2 x2+9C3 x3+...+ 9C7 x7+...) Product of getting x7 Here 1×9C7 x7 and −x4 9C3 x3 ∴9C7−9C3