x^2-2x>=0
x=2
假设√(x^2-2x)=t>=0
y=3x^2-6x-2√(x^2-2x)
=3(x^2-2x)-2√(x^2-2x)
=3t^2-2t
=3(t-1/3)^2-1/3
0=-1/3