1) x^2-4x+3=0
x^2-4x-4+4+3=0
(x^2-4x+4)-4+3=0
(x-2)^2-1=0
(x-2-1)(x-2+1)=0
x-3=0
x1=3
x-1=0
x2=1
2) x^2-6x+5=0
x^2-6x-9+9+5=0
(x^2-6x+9)-9+5=0
(x-3)^2-4=0
(x-3-2)(x-3+2)=0
x-5=0
x1=5
x-1=0
x2=1
3)x^2+8x-20=0
x^2+8x-16+16-20=0
(x^2+8x+16)-16-20=0
(x+4)^2-36=0
(x+4-6)(x+4+6)=0
x-2=0
x1=2
x+10=0
x2= -10
4)x^2+12x+32=0
x^2+12x-36+36+32=0
(x+6)^2-4=0
(x+6-2)(x+6+2)=0
x1= -4
x2= -8
5) x^2-2x-15=0
x^2-2x-1+1-15=0
(x-1)^2-16=0
(x-1-4)(x-1+4)=0
x1=5
x2= -3