1.x^2-6x=-8
x^2-6x+9=-8+9
(x-3)^2=1
x-3=±1
解得x1=4 x2=2
2.x^2-8x-4=0
x^2-8x+16-16-4=0
x^2-8x+16=20
（x-4）^2=20
x-4=±2√5
解得x1=（2√5）+4 x2=4-（2√5）
3.x^2+3x+2=0
x^2+3x+9/4-9/4+2=0
(x+3/2)^2=1/4
x+3/2=±(1/2)
解得x1=-1 x2=-2
4.-x^2+5x+6=0
-（x^2-5x）+6=0
x^2-5x=6
x^2-5x+25/4=6+25/4
(x-5/2)^2=49/4
x-5/2=±(7/2)
解得x1=6 x2=-1