# Deriving Lagrange's Trig Identity (real part of a complex number in exponential form)

1. May 12, 2010

### Lchan1

1. The problem statement, all variables and given/known data
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?

2. Relevant equations

3. The attempt at a solution

2. May 12, 2010

### Staff: Mentor

Re: Deriving Lagrange's Trig Identity (real part of a complex number in exponential f

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

3. May 12, 2010

### Lchan1

Re: Deriving Lagrange's Trig Identity (real part of a complex number in exponential f

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.

4. May 13, 2010

### Staff: Mentor

Re: Deriving Lagrange's Trig Identity (real part of a complex number in exponential f

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