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Three rules Sum rule: finite disjoint sets S and T SUT=S+T Product rule: finite sets S and T |S×TI=IS1·IT到 Bijection rule: finite sets S and T 3φ：S 1-1T→lS1=T到 on-to Three rules Product rule: |S ⇥ T| = |S|·|T| finite sets S and T Sum rule: finite disjoint sets S and T |S T| = |S| + |T| Bijection rule: finite sets S and T ⇤ : S 11 ⇥ onto T = |S| = |T|