因为根号开出来是非负数,所以y≥x
x^2-3x+2≥0
x≥2或x≤1
x≥2时,y=x和y=根号（X^2-3X+2)都是增函数,所以y≥2
x≤1时,y=x是增函数,y=根号（X^2-3X+2)是减函数,所以要单独算
y-x=根号（X^2-3X+2)
(y-x)^2=x^2-3x+2
y^2-2xy+x^2=x^2-3x+2
y^2-2xy+3x-2=0
y^2+(3-2y)x-2=0
x=(2-y^2)/(3-2y)≤1
y＜3/2时,2-y^2≤3-2y,y^2-2y+1≥0,此时不等式恒成立
y>3/2时,2-y^2≥3-2y,y^2-2y+1≤0,此时不等式无解
所以y