y=（x-2）的三次方-x的立方
=(x-2-x)[(x-2)^2+(x-2)*x+x^2]
=-2(x^2-4x+4+x^2-2x+x^2)
=-6x^2-6x+4
=-6(x+1/2)^2+2.5
当x=-1/2时,y最大是2.5