Ok, here's the deal. I've been trying to graph the function x^2+8x+y^2-8y+2xy+16=0, and on a graphing calculator such as this here, and I've noticed the graph is a circle, with centre (-4,4) and radius 4. x^2+8x+y^2-8y+2xy+16 seems to be equivalent to x^2+8x+y^2-8y+16, which is also a circle...