An area proof for the Law of Cosines when C > 90° is exhibited in the figure below (fou...

An area proof for the Law of Cosines when C > 90° is exhibited in the figure below (found in H. Eves, Survey of Geometry, Vol. I. p. 268). Analyze the figure and provide a proof. (From trigonometry, the standard area for a parallelogram with two sides of length a and b and the angle between having measure θ is A = ab sin θ.)