I'm trying to develop the algebraic proofs for these two formulas that appear on the webpage below! The image below is of an unfolded non-regular tetrahedron. Triangle B represents the dihedral angle between the planes of triangle C and triangle E. The other triangles represent the three faces and the base triangle.
These are the two formulas I'm trying to prove:
Angle C = $\arccos\big(\frac{\cos(A)}{cos(B)}\big)$
Angle B = $\arcsin(\cos(D) × \sin(A))$
The author uses a unit circle to represent the Hypotenuse of the various triangles.
Here's the link: http://sbebuilders.blogspot.com/2014/09/trigonometric-formulas-geometrically.html
Note: the images are from the web page, and are the work of the webpage author.