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Question

If the coefficients of x3 and x4 in the expansion of (3+kx)9 are equal, then the value of k is

A
3
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B
1/3
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C
2
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D
1/2
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E
1
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Solution

The correct option is C 2
In expansion (3+kx)9,
coefficients of x3 and x4 are equal
9C3(3)6(k)3=9C4(3)5(k)4[Tr+1=nCranrbr]
9!3!6!(3)=9!4!5!×k
k=3×4×3!×5!3!×6×5!
k=126=2

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