(3x²-2x+7)-(4x²-2x+7)
=3x²-2x+7-4x²+2x-9
=-x²-2
因为x²≥0
-x²≤0
所以-x²-2(3x²-2x+7)-(4x²-2x+7)=3x²-2x+7-4x²+2x-9？因该是(3x²-2x+7)-(4x²-2x+7)=3x²-2x+7-4x²+2x-7吧！哦，对不起(3x²-2x+7)-(4x²-2x+7)=3x²-2x+7-4x²+2x-7=-x²因为x²≥0所以-x²≤0所以3x²-2x+7≤4x²-2x+7