# Homework Help: Express the following in the form of a Complex Number

1. Sep 18, 2013

### Minhq604

1. The problem statement, all variables and given/known data
For my waves class, I have to do this problem. I've previously completed a question like this except there was no phase constant (∏/4) in that question.
Express the following in the form x = Re [Ae^i$\alpha$e^iwt
x=cos(wt + ∏/4) - sin(wt)

2. Relevant equations

euler's formula e^iwt = cos(wt) + isin(wt)
complex amplitude = √A2+B2
complex angle = tan $\alpha$ = y/x

3. The attempt at a solution

I know that cos(wt + ∏/4) = Re [e^i(wt + ∏/4)] and -sin(wt) = Re [ie^iwt]

combining these two, i have Re [e^i(wt + ∏/4) + ie^iwt] which, after factoring, becomes
Re [e^iwt (e^i∏/4 + i)]

My problem is, what do i do with the e^i∏/4 to get the complex amplitude and complex angle? In my previous problem without the ∏/4 shift, i was able to plot in Cartesian coordinates Im vs Re and successfully convert to polar coordinates. Help?

2. Sep 18, 2013

### voko

$e^{i \pi/4}$ is a complex number. So is $e^{i \pi/4} + i$. Find it, then represent it as $A e^{i\alpha}$.

3. Sep 18, 2013

### rude man

How about changing your x into x = Acos(x + ψ)? Just high school trig.

Last edited: Sep 19, 2013
4. Sep 18, 2013

### Minhq604

so i made the e^i$\pi$/4 into cos($\pi$/4)+sin($\pi$/4) by using eulers formula. Now i calculated the complex amplitude to be √2.414. and the complex angle to be 35.26 degrees or 0.6155 rad. The answer i get is x = Re [√2.414 e^i0.6155t e^iwt ] is this correct?

5. Sep 18, 2013

### voko

This is the complex amplitude of what?