# Using Euler derive the relationships between cosine and exponential function [closed]

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

## closed as off-topic by Siong Thye Goh, Henrik, JonMark Perry, Claude Leibovici, José Carlos SantosSep 19 '17 at 8:59

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