If nCr–1 = nC3r, find r.

Given: nCr–1 = nC3r


To find: r=?


We know that:


nCr = nCn-r


⇒ nCr–1 = nCn-(r-1)


⇒ nCr–1 = nCn-r+1


⇒ nCn-r+1= nC3r


⇒n-r+1=3r


⇒4r=n+1


⇒


Ans:


1