解;
令√x+2=t
x+2=t^2
x=t^2-2
y=2t-3(t^2-2)
=2t-3t^2+6
=-3t^2+2t+6
=-3(t-1/3)^2+19/3
因为t≥0
所以最大值是19/3
没有最小值