Deriving Lagrange's Trig Identity: Real Part of Complex # in Exp Form

[Lchan1]

Homework Statement

I did the question with help, but did not understand why did we multiply e^-i(x/2)
How do I know what to multiply for getting the real part of a complex number in exponential form?

Homework Equations

The Attempt at a Solution

 

[Mark44]

Homework Statement

I did the question with help, but did not understand why did we multiply e^-i(x/2)
How do I know what to multiply for getting the real part of a complex number in exponential form?

Homework Equations

The Attempt at a Solution

You need to give us more information. I don't know what problem you're trying to solve.

 

[Lchan1]

http://mathforum.org/library/drmath/view/64574.html

instead of using euler's formula i multiply by e^-(x/2)/e^-(x/2)
after letting z=e^ix

I am sorry I was in a rush at school.

 

[Mark44]

Show us what you're doing and where you're getting stuck.