Example: The sets Z,Q, with the usual operations of multiplication and addition form rings, [Z;+,×],Q+,× are rings Let m=((aiinxnlai is real number, Then M; +, x is a ring ◆ Example:S≠,P(S):,n], ◆ Commutative ring Example: The sets Z,Q, with the usual operations of multiplication and addition form rings,  [Z;+,],[Q;+,] are rings  Let M={(aij)nn |aij is real number}, Then [M;+,]is a ring  Example: S,[P(S);,∩]，  Commutative ring