太简单了.
把两条式子作差运算就行.
(3x²-2x-7)-(4x²-2x+5)
=3x²-2x-7-4x²+2x-5
=3x²-4x²-12
=-x²-12
=-(x²+12)
因为x² ≥ 0,则x²+12 ≥ 12,则-(x²+12)≤-12
也就是说,(3x²-2x-7)-(4x²-2x+5)结果是一个负数,所以(3x²-2x-7) ＜ (4x²-2x+5)