## anonymous 4 years ago Prove that if m and n are positive integers: 1≤k≤n, then $\sum_{i=0}^{k}(n choose i)(mchoosek-i)=(m+n choose k)$

1. anonymous

i don't know how to write those notation with ("n" the top and "i"under it)

2. anonymous

can you draw the equation instead?

3. anonymous

I am going to attach it.

4. anonymous

Hope you are able to see it

5. anonymous

Do you know what combinatorial proofs are?