1)x^2 5x 7=(x^2 5x (5/2)^2) 7-(5/2)^2=(x 5/2)^2 3/4
(x 5/2)^2>=0,且在x=-5/2时取到最小值0,因此,原式最小为3/4
2)2x-2x^2-1=-2(x^2 x)-1=-2(x^2 x (1/2)^2)-1 2*(1/2)^2=-2(x 1/2)^2-1/2
-2(x 1/2)^2