(1)x^2-12x-4=0
(x-6)^2-36-4=0
(x-6)^2=40
x-6=+-2*10^1/2
x=6+-2*10^1/2
(2)x^2+3x-4=0
(x+3/2)^2-9/4-4=0
(x+3/2)^2=25/4
x+3/2=+-5/2
x=+-5/2-3/2
x1=1,x2=-4
3)2x^2-5x-1=0
x^2-5/2x-1/2=0
(x-5/4)^2-25/16-1/2=0
(x-5/4)^2=33/16
x-5/4=+-33^1/2/4
x=5/4+-33^1/2/4=(5+-33^1/2)/4