x^2-20x+20=0
x^2-20x+19+1=0
(x-1)(x-19)+1=0
(x-1)(x-19)=-1
(1-x)(x-19)=1
x^2=(x-4)^2+(x-2)^2
x^2=(x-2-2)^2+(x-2)^2
x^2=2(x-2)^2-4x+8+4
2(x-2)^2-(x^2+4x+12)=0
2(x-2)^2-(x-2)(x+6)=0
(x-2)[2(x-2)-(x+6)]=0
(x-2)(x-10)=0