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Question

If  nC2+ nC3= n+1Cr, then the minimum possible value of r is 
  1. 0
  2. 3
  3. 2
  4. 1


Solution

The correct option is D 1
Given  nC2+ nC3= n+1Cr
We know that, 
 nCr+ nCr1= n+1Cr
So,
 nC2+ nC3= n+1Cr n+1C3= n+1Crr=3 or r+3=n+1r=3,n2
From the given expression we know that, 
n3
So, the minimum possible value of r is 1 when n=3.

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