Teaching History of Math this past semester gave me an excuse to read carefully two Dover Publications books that I have owned since high school, but only skimmed then. Imagine my delight to discover that if you are given a theorem that is hard to prove beforehand, you can prove that 
is transcendental in just a couple of lines. The hard theorem gives many other corollaries too, corollaries that I’ve known in my gut but never had a handle on how to prove.
Here are the details, from p. 76 of Felix Klein’s book Famous Problems of Elementary Geometry. You can read it on-line at Google Books.
Theorem (Lindemann): Over the complex field, in the equation 
not all of the 
and 
can be algebraic, assuming that at least one 
Corollary 1:
is transcendental.
Proof:
and 1 is trivially algebraic, so 
is transcendental, so then also is 
Corollary 2: In the equation 
if x is algebraic and 
then y is transcendental. If x is a non-zero rational multiple of 
then y is algebraic.
Proof: Note first that by a power series argument, for example, 
Therefore, ~~ 2 i \sin {x} - e^{i x} + e^{-i x} ~=~ 0.)
If x is algebraic then, Lindemann’s theorem applied to (*) shows that
is transcendental. If x is a rational multiple of then x is transcendental in a simple proof by contradiction from Corollary 1, hence y cannot be transcendental, again by Lindemann’s theorem.
Similarly, we can show that in 
only one of x or y can be algebraic (excluding the y = 0 case, as Lindemann’s theorem does, because 
). Similarly for all of the rest of the trig functions and inverse trig function, which can also be expressed as rational functions of exponentials.
And finally, what is the heart of the proof of Lindemann’s theorem in the first place? It rests on the fact that 
as a power series has factorials in denominators, making any attempt to make it satisfy an algebraic equation a failure.
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