$e^{i2\pi a}=1$ or $e^{i2\pi a}=\cos(2\pi a)+i\sin(2\pi a)$?

Question

Since $e^{i2\pi a}=(e^{i2\pi})^a=(1)^a=1$ but $e^{i2\pi a}=\cos(2\pi a)+i\sin(2\pi a)\neq1$, why is that?