Hi, I have to hand in my homework in two days and I have no idea how to prove the following (freely translated, English is not my native language so hope it makes sense):

"Show that for all positive whole numbersnandk,whennis bigger than or equal tok,the following applies:choose (

(nk-1))+(nchoosek)=((n+1) choosek)

My textbook uses the word choose, not sure if that's universal so "n choose k"=n!/k!(n-k)! so you know what I'm talking about.

Any help greatly appreciated!