当x<4-x时可得x<2此时当x>1时g(x)=f2(x)=1当x<1时g(x)=f1(x)=x当x>2时g(x)=f3(x)=4-x可得g(x)=f1(x)=x(x<1) g(x)=f2(x)=1(1= g(x)=f3(x)=4-x(x>2)最大值是当x=2时g(x)=2