x^3-(x^2-x+1)
=x^3-x^2+x-1
=(x^3-x^2)+(x-1)
=x^2(x-1)+(x-1)
=(x^2+1)(x-1)
因为x^2+1>0,x-1≥0
所以x^3-(x^2-x+1)≥0
x^3≥x^2-x+1