Fundamental Theorem of Trigonometry This is a pretty open ended question and I apologize, in advance, if this is not the place for it.  But what do you recommend should be given the title of the Fundamental Theorem of Trigonometry and why?  Should we have to restrict ourselves to the planar case... I think so.
$$\boxed{\sin^2x+\cos^2x=1.}$$
$$\frac{\sin x}{\cos x} = \tan x$$
$$sin^2 x + cos^2x = 1$$