该题结论不对,若g.f是满射的,则g是满射,不能得f是满射.如A={1,2},B={a,b,c},C={5,6},
f:A->B,f(1)=a,f(2)=b,
g:B->C,g(a)=5,g(b)=6,g(c)=6,
g.f(1)=g(a)=5,g.f(2)=g(b)=6,显然复合函数g.f是满射的,但f不是满射.