Find the (r+1)th term in the expansion of (x+y)n
nCrxn−ryr
There will be (n+1) terms in the expansion of (x+y)n
The first term will be nC0xny0 and the last term or (n+1)th term will be nCnx0yn. If you observe more terms, you will find out that
Tr+1=nCrxn−ryr