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Q.

 If the coefficients of rth term and (r+1) th terms in the expansion of (1+x)20are in the ratio 1:2, then r=

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a

6

b

7

c

8

d

9

answer is B.

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Detailed Solution

The ratio of coefficient of rth term and (r+1)th term=1:2

= 20Cr−1⋅20Cr =12⇒r21−r=12⇒2r=21−r⇒r=7

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