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
r=8
So,
rC5= 8C5
⇒8C5=56
Ans: 8C5=56