>>129
ax^2+bx+c=a(x^2+(b/a)x)+c
=a(x^2+(b/a)x+(b/2a)^2-(b/2a)^2)+c
=a(x^2+(b/a)x+(b/2a)^2)-b^2/4a+c
=a(x+b/2a)^2-b^2/4a+c