f'(x)=x^2-3x+2.
ax^2-3x+a+1>x^2-x-a+1,
a(x^2+2)>x^2+2x
a>(x^2+2x)/(x^2+2)
因为(x^2+2)>0,任意a属于(0,正无穷)都成立,所以(x^2+2x)=