So here is my question, I understand how to derive the cosine rule from, both triangles acute and obtuse. My problem is the 3 formula you get from this equation. When I derive from a triangle I get the formula: [itex]c^2=a^2+b^2-2abcos∅[/itex] so how do you derive the other two formula, I read that the letter are interchangeable so you can place them where every you like on the triangle. If that is the case, why do we show the other two formula? I presume people reading this post know of the other two formula I am referring to if not here they are: [itex]a^2=c^2+b^2-2cbcos∅[/itex] [itex]b^2=a^2+c^2-2accos∅[/itex]. This has been on my mind for a while, I would appreciate if someone could clarify my confusion. Big thanks in advance.