n−r+1r\frac{n-r+1}{r}rn−r+1
n−rr\frac{n-r}{r}rn−r
n−r−1r\frac{n-r-1}{r}rn−r−1
nCr+1nCr\frac{{ }^{n} C_{r+1}}{{ }^{n} C_{r}}nCrnCr+1