# Find the zeroes of r. What to do when it is a multiple of theta?

1. Apr 24, 2015

### Tyrion101

I understand what to do when theta is by itself such as r=5+10cos(theta) but what if you have the same equation, and it is (2theta) I am on taptalk by the way.

2. Apr 25, 2015

### Staff: Mentor

What have you tried?
In essence, you're trying to solve the equation $5 + 10\cos(2\theta) = 0$.

Last edited: Apr 25, 2015
3. Apr 25, 2015

### Tyrion101

Well if it were without the 2*theta I'd set it equal to 0, like you did, and solve for cos(theta). Well anyway without the 2 you turn the equation into cos=-1/2, would I just multiply -1/2 by 2? Or would I add 2pi or something?

4. Apr 25, 2015

### symbolipoint

Solve for 2*theta. Solve for theta from that.

5. Apr 25, 2015

### Staff: Mentor

You took a trig class, right? Do you still have your textbook? If so, there should be numerous examples of solving trig equations, including ones that involve the sine or cosine (or other trig function) of angle multiples.