Theorem(Sum Principle) SnT=0→ISUT=S1+1Tl Theorem (Product Principle) IS×T=IS×T Holds for finite sets S and T. Hengfeng Wei (hfweixinju.edu.cn) 2-3 Counting March12,20209/34Theorem (Sum Principle) S ∩ T = ∅ =⇒ |S ∪ T| = |S| + |T| Theorem (Product Principle) |S × T| = |S| × |T| Holds for finite sets S and T. Hengfeng Wei (hfwei@nju.edu.cn) 2-3 Counting March 12, 2020 9 / 34