Understanding Vandermonde's Identity: Explanation and Proof

[Chromium]

Hey guys,

Could someone give me a clear explanation of what Vandermonde's Identity is? I'm looking at the proof in my book and I'm having a difficult time understanding this. Fortunately I understand the rest of the section (which covers Binomial theorem, Pascal's identity and triangle).

thanks

 

[durt]

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Could you be more specific as to what you don't get? The proof is rather straightforward.

 

[neutrino]

Off-Topic: Every time someone post's a thread on this topic, I keep wondering why people talk about Harry-Potter-stuff on the maths forums.