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## Main Question or Discussion Point

So it's apparently possible to prove that e^{-iAx} = cos(x)I + isin(x)A given that A^2=I.

What I don't understand is how this is supposed to be derived. Any help would be appreciated as this is driving me nuts and this is probably something that is very easy to prove...

What I don't understand is how this is supposed to be derived. Any help would be appreciated as this is driving me nuts and this is probably something that is very easy to prove...