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Question

If r213r+40=0, then the value of 7Cr is

A
56
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B
12
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C
1
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D
21
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Solution

The correct option is D 21
Given : r213r+40=0
(r8)(r5)=0
r=8,5
We know, nr for nCr
So, only possible value of r=5
7Cr= 7C5
7C5=7!2!5!=765!215!
7C5=21

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