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Question

If the ratio of the coefficients of rth term and (r+1)th term in the expansion of (1+x)20 is 1 : 2

then r =?

A
4
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B
5
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C
6
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D
7
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Solution

The correct option is D 7
Let the two successive terms be Tr and Tr+1.
Therefore it is given that
TrTr+1=12
or
2Tr=Tr+1.
220Cr1=20Cr
220(r1)=1r
2r=20r+1
3r=21
r=7
Hence the two consecutive terms are T7 and T8

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