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Emeritus
 Quote by dimension10 I think this is not possible. Maybe your friend considers $$\sin\theta=\frac{{e}^{i\theta}-{e}^{-i\theta}}{2i}$$ $$\cos\theta=\frac{{e}^{i\theta}+{e}^{-i\theta}}{2}$$ to be definitions of sine and cosine?