1.
x^2+3x=0
(x+1.5)^2=2.25
x+1.5=1.5或x+1.5=-1.5
x=0或x=3
2.
x^2+x-1=0
(x+0.5)^2=5/4
x+0.5=±√5/2
x=-0.5±√5/2
3.
x^2-(7/2)x+3/2=0
(x-7/4x)^2-49/16+3/2=0
(x-7/4)^2=25/16
x-7/4=5/4或x-7/4=-5/4
x=3或x=1/2
4.
x^2-4x+4.5
=(x-2)^2+0.5≥0.5＞0
5.
只需证明二次项系数不等于0
a^2-8a+20
=(a-4)^2+4≥4＞0
二次项系数不等于0,则方程恒为一元二次方程