Euler's Identity problem?

Question

Show that cos(theta) = ( e^(i*theta) + e^(-i*theta) )/2

Answer isn't as important as knowing how to do it. A simple link would suffice, but a though out explanation is helpful too.

Answer 1

e^(i*theta) = cos(theta) + i*sin(theta)

That equation is an identity (Euler's identity)

If you use that then:

e^(-i*theta) = cos(-theta) + i*sin(-theta)

Also, you need to know that cos(theta) = cos (-1*theta) and sin(theta) = -1*sin(-theta)

If you use those identities, you'll be golden.

Hint, the imaginary parts will cancel out (1i - 1i).