Senior three, I know, I know. Look back at senior one and senior two, it's too simple!

The sense of achievement in learning mathematics is that you can't think of it if you try hard, and you can get it inadvertently! ! ! ! !

Think more, draw more pictures, and consider why empty sets exist.

Look at the definition! ! ! !

Don't recite axiomatic formulas, deduce them yourself! ! ! ! !

Ok, if you keep doing this, you will certainly make progress! ! !

Trust you!