I am gonna go crazy about this. In Skecth 14, even though I put the same dimensions and angles on both sides, it doesn't seem to be symmetric exactly. The two lines from the bottom corners of the rectangle to the curve that I am working on are not exactly the same. They are both 45 degrees to the bottom line which is horizontal, and coincident to the curve, but their lenghts are not the same. You might think of first sketching only one side and make a mirror of it but I want to understand what causes it! Please help!