At the moment I a trying to really get to grips with these rules proof wise. I would like to know if I am on the right track in learning the proof correctly. In my books it goes through the proof in breaking up an acute and obtuse triangle and the solving via trigonometry/Pythagoras. I understand all right until the part on an obtuse triangle until it gets to the - 2bc cos part, I took that from the c2 formulae of cosine. I don't really want anyone telling me why it like this more, am I going in the right direction, by looking a the proof via a unit circle, and looking in the negative x quadrant? My books do not show this proof nor do they show the unit circle, as like most per degree exams, I don't need to show the proof just remember the formulae and apply the correctly. So am I heading in the right direction in understanding the proof?