The Law of Cosines. This worksheet provides a visual proof of the Law of Cosines: [math] c^2=a^2+b^2−2ab \cos C[/math], or[br][math]a^2=b^2+c^2−2bc \cos A[/math], or[br][math]b^2=a^2+c^2−2ac \cos B[/math].[br]Although this sheet always proves using the first form by placing [math]\angle C[/math] in the standard position, the fact that you can relabel the same triangle and get the same results shows that the other two forms are also true.