# Homework Help: Complex Numbers and amplitude

1. Mar 9, 2010

### mmmboh

Hi this isn't homework, just a practice problem I already have the answer too for my waves class:
z=sin(wt)+cos(wt)
Express this in the from Z=Re[Aej(wt+$$\alpha$$)]

I know how to express sine in the form of cosine, and cosine in the from of a complex exponential, but I don't know how to do this...I need to find the amplitude and $$\alpha$$. Can someone help?

2. Mar 9, 2010

### vela

Staff Emeritus
Expand $Re[Ae^{j(\omega t+\alpha)}]$ in terms of sin(ωt) and cos(ωt) and compare it to z.

3. Mar 9, 2010

### mmmboh

Well for $Re[Ae^{j(\omega t+\alpha)}]$...the inside equals$Ae^{j(\omega t)}e^{j\alpha}[/tex] and [itex]e^{j\omega t}$=cos(wt)...I'm not really sure where to go from there.

4. Mar 9, 2010

### vela

Staff Emeritus
You need to get the inside into the form x+iy so you can just pick off the x when you take the real part. Don't break the exponential up. Just use Euler's formula on it.

5. Mar 9, 2010

### mmmboh

Ok so cos(wt)+sin(wt)=Re[Acos(wt+a)]+Re(Ajsin(wt+a)....and now..I don't really get what the Re does, I know that means real, but what is the significance of it here? Am I suppose to take out the j or something?

6. Mar 9, 2010

### vela

Staff Emeritus
You throw away the imaginary part: Re[x+iy]=x.

7. Mar 9, 2010

### mmmboh

Oh I got it thanks!