Answer :

Given: 20Cr = 20Cr+6


To find: r=?


We know that:


nCr = nCn-r


20Cr+6= 20C20-(r+6)


20Cr+6= 20C20-r-6=20C14-r


20C14-r= 20Cr


14-r=r


2r=14



Ans:r=7


ii) Given: 18Cr = 18Cr+2


To find: rC5=?


We know that:


nCr = nCn-r


18Cr+2= 18C18-(r+2)


18Cr+2= 18C18-r-2=18C16-r


18C16-r= 18Cr


16-r=r


2r=16



So,


rC5= 8C5


8C5=


8C5=


8C5=


8C5=


8C5=


8C5=56


Ans: 8C5=56


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