Permutations and Combinations 2 Question 25

25. If nCr1=36,nCr=84 and nCr+1=126, then find the values of n and r.

(1979,3 M)

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Solution:

  1. We know that, nCrnCr1=nr+1r

8436=73=nr+1r3n10r+3=0

Also given, nCrnCr+1=84126

r+1nr=232n5r3=0

On solving Eqs. (i) and (ii), we get

r=3 and n=9



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