Classical probability lottery problem

You can understand it as putting the ball of a+b in the small box of a+b, and how many problems there are with putting the red ball in the kth box.

First, choose a red ball to put in the K, there is a red ball to choose from, and then how to put the rest of the balls has nothing to do with this K box. You can guarantee that it will be red when you take it out. So the rest of the ball can be released (a+b- 1)! Sowing and releasing methods.

Divided by all the balls, it is (a+b) balls, and one ***(a+b)! Kindness

I don't know if you can understand the train of thought of this problem in this way.