y=sqrt(x^2+1)+Sqrt((4-x)^2)+4
y1=sqrt(x^2+1),y2=Sqrt((4-x)^2)
Min(y1)时,X=0,Min(y1)=1,Y2＝4,Y＝9
Min(y2)时,X=4,Min(y2)=0,Y1＝5,Y＝9
所以,Min(Y)=9