  Taking up the challenge McConnell laid down at the end of his proof of the
law of cosines, we give a completely visual dissection proof of this theorem,
which applies to any triangle. In order to avoid the trigonometric expressions
of Cuoco-McConnell's proof, we replaced the equal-area rectangles with
congruent triangles. As a matter of fact, trigonometric expressions are
implicitly based on the similarity of two right triangles with a common
non-right angle. So they are conceptually less simple than our congruent
triangles which are, moreover, easy to visualize. This makes our proof the only
dissection proof and the simplest proof of its family, and thus one of the best
options for a course of geometry.
