Hi, I am teaching myself the discrete math using the "Discrete Mathematics Lecture Notes, Yale University, Spring 1999". And I don't how to prove the practice question below: Prove that |A∪B|+|A∩B| = |A|+|B|. The text book provide answer like below: The common elements of A and B are counted twice on both sides; the elements in either A or B but not both are counted once on both sides. But I think it may be some mathematical way to prove that. If someone can give me a hand that will be very appreciated.