how to prove that
[tex] \cos( \alpha ) = \frac{1}{2} ( {e}^{i \alpha } + {e}^{ - i \alpha } )[/tex]


Answer :

First you need to do is solve a by simplifying both sides.
After you’ve done that you would need to keep the variable alone.
e
i
a
2
+
e

i
a
2

cos
(
a
)
=
0