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Question

Find a positive value of m for which the coefficient of x2 in the expansion of (1+x)m is 6.

A
2
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B
4
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C
6
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D
8
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Solution

The correct option is B 4
We know that the coefficient of xr in (1+x)n is nCr.

Therefore, the coefficient of x2 in (1+x)m is mC2.

It is given that the coefficient of x2 in (1+x)m is 6.

Therefore, mC2=6

m(m1)2!=6

m2m=12

m2m12=0

(m4)(m+3)=0

m4=0

m=4

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