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The value of r for which 20Cr 20C0 + 20Cr–1 20C1 + 20Cr–2 20C2 +.........+ 20C0 20Cr is maximum is:

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The value of r for which 20Cr 20C0 + 20Cr–1 20C1 + 20Cr–2 20C2 +.........+ 20C0 20Cr is maximum is: 

(1) 20 

(2) 10 

(3) 11 

(4) 15

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1 Answer

+2 votes
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Best answer

The correct option is (1).

Explanation:

Given sum = Co. of xr in the expansion of (1 + x)20 (1 + x)20

i.e. (1 + x)40 

which can be given as 40Cr .

It is maximum at r = n/2 = 40/2 = 20

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