（1）x²-3x+4=(x-1.5)²+1.75∵(x-1.5)²≧0∴x²-3x+4=(x-1.5)²+1.75＞0∴x²-3x+4的值永为正数（2）-2x²+3x-2=-2（x²-3/2x+9/16－9/16)-2=-2(x-3/4)²-7/8＜0∴-2x²...