（1）0.1x^2-0.4x-1.2=0
x^2-4x-12=0
(x-6)(x+2)=0
x-6=0,x+2=0
x1=6,x2=-2
（2）0.5x^2+3.5x+5=0
x^2+7x+10=0
(x+2)(x+5)=0
x+2=0,x+5=0
x1=-2,x2=-5