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Question

If the coefficient of (2r+3)th term and (r−1)th terms in the expansion of (1+x)18 are equal, find r.

A
r=6
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B
r=7
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C
r=11
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D
r=12
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Solution

The correct option is A r=6
Since the two coefficients are equal
18Cr2=18C2r+2
18(r2)=2r+2
18+2r=2r+2
3r=18
r=6

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