1)當x>1時,
x^3-(x^2-x+1)=x^3-x^2+x-1
=(x-1)(x^2+1)
>0.
因此 x^3>x^2-x+1.
2)因為 (x^2+y^2+1)-2(x+y-1)
=x^2+y^2-2x-2y+3
=(x^2-2x+1)+(y^2-2y+1)+1
=(x-1)^2+(y-1)^2+1
>0.
因此 x^2+y^2+1>2(x+y-1).
= = = = = = =
比較大小的,用作差法或作比法就行.