Proof:(1) For any a∈A, there exists c∈ C such that(a2c)∈fg? 令(2) For every a∈A, If there exist x, y∈ C such that(a,x)∈ fogang(a2y)∈∫g, then x=y? o Definition 3.4: Let g be a(everywhere) function from A to B, and be a(everywhere) function from B to C. Then composite relation f og is called a(everywhere) function from A to O, we write og:A→C.Ifa∈ then(°g)(a)=f(g(a)❖ Proof: (1)For any aA, there exists cC such that (a,c) f g? ❖ (2)For every aA, If there exist x,yC such that (a,x) f gand (a,y) f g，then x=y? ❖ Definition 3.4: Let g be a (everywhere) function from A to B, and f be a (everywhere) function from B to C. Then composite relation f g is called a (everywhere) function from A to C, we write f g:A→C. If aA, then(f g)(a)=f(g(a))