Reduction yes instance a polynomial-time instance B polynomial-time >yes ofA reduction algorithm of B algorithm to decide B no no polynomial-time algorithm to decide A it provides us a way to solve problem A in polynomial time: 1.Given an instance o of problem A,use a polynomial-time reduction algorithm to transform it to an instance B of problem B. 2.Run the polynomial-time decision algorithm for B on the instance B. 3.Use the answer for B as the answer for a.Reduction