Q15 of 64 Page 122

If nCr – 1 = 36, nCr = 84 and nCr + 1 = 126, then find rC2.

[Hint: From equation using to find the value of r.]

Formula:- (i)nCr


Given:-


nCr-1=36,nCr=84,nCr+1=126




2n-2r=3r+3


⇒2n-3=5r-------------------(i)




=3n-3r+3=7r


3n+3=10r-----------------(2)


From (1) and (2)


2(2n-3)=3n+3


4n-3n-6-3=0


n=9


and r=3


now


rC2=3C2


=3


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