Using Euler derive the relationships between cosine and exponential function

Question

$$\cos\phi=\frac{1}{2}(e^{j\phi} + e^{-j\phi}).$$ Please i was told to do this assignment but cant prove if anyone can help me out

Answer 1

I would write out what is $\frac{1}{2}(e^{ix} + e^{-ix})$ using euler's formula, and see if anything cancels. hint: something cancels

Answer 2

Hint: $$e^{j\phi}=\cos\phi+j\sin\phi$$