1,
4m-5-m^2=-(m-2)²-1≠0;
△=(2m-1)²-4*(4m-5-m²)*1
=8m²-20m+21
=2[(4m²-10m+25/4)-25/4]+21
=2(2m-5/2)²-25/2+42/2
=2(2m-5/2)²+17/2>0;
无论m任何实数时,关于x的方程(4m-5-m^2)x^2+(2m-1)x+1=0都是一元二次方程;
2,
-2x^2+5x+7
=-2( x²-5x/2+25/16-25/16)+7
=7-2(x-5/4)²+25/8
=81/8-2(x-5/4)²
-2(x-5/4)²<=0
当x=5/4时,-2(x-5/4)²=0,
代数式-2x^2+5x+7有最大值81/8;
3,
1/2 x^2-2x+1
=(1/2)(x²-4x+4-4)+1
=(1/2)(x-2)²-2+1
=(1/2)(x-2)²-1
(1/2)(x-2)²>=0,
当x=2时,(1/2)(x-2)²=0,
代数式1/2 x^2-2x+1的最小值-1;
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