1).(x-1)(4-x)》0,1《x《4.x求根取正,反函数取正.x=1时,y最小=3,y>3即反函数x>3.2).y-3x=根号[（x-1)(4-x)],平方,y^2-6yx+9x^2=-x^2+5x-4.10x^2-(6y+5)x+y^2+4=0.求根:x=[6y+5+根号（6y+5)^2-40(y^2+4)]/20=[6y+5+根号（-4y^2+60y-135）]/20.x、y对换,y=[6x+5+根号(-4x^2+60x-135)]/20,x>3.