# Homework Help: Find theta angle in complex form

1. Mar 15, 2015

### baby_1

1. The problem statement, all variables and given/known data
Here is my equation that I want to find theta angle

2. Relevant equations

3. The attempt at a solution
I try to set different value of cos (theta) to find theta but it failed , I want to know the main solving strategy
Thanks

2. Mar 15, 2015

### RUber

What if you were to substitute cos(theta) = x and sin(theta) = y, and enforce the fact that y = sqrt(1-x^2)? Usually these eventually simplify to a quadratic equation to solve--after some work.

3. Apr 8, 2015

### baby_1

I did with the same way:

as I plot these charts I have

It has two roots (+.63,-.63) but the both of them indicate that theta should be 53 degree that is opposite the my books answer that theta should be 12 degree

what is my fault?

4. Apr 8, 2015

### BvU

No fault. I don't get 53 degrees but 50.9612 (cos = 0.629847 so like you found it).

Looks as if the book answer is wrong.

5. Apr 9, 2015

### RUber

I concur with BvU. 12 degrees does not satisfy the equation.
cos(pi/2*cos( 12 ))/sin( 12 ) = 0.1651,which does not equal sqrt(2)/2 = 0.7071.