# Finding angle inverse cosine

1. Oct 1, 2015

### Mechaman

• Member warned about posting without the HW template
Not sure if I'm doing this right. I have an angle theta to find but the cosine has been squared. I brought over inverse cosine to multiply to leave theta on its own. I was told the answer should be closer to 37 degrees? Am I doing something wrong here?

2. Oct 1, 2015

### Staff: Mentor

Did you calculate it via

arccos(sqrt(50/79.57) ?

3. Oct 1, 2015

### Staff: Mentor

On the 4th line from the bottom you have $\frac{\sigma_n}{\sigma_x} = \cos^2(\theta)$
This is equivalent to $\cos(\theta) = \pm \sqrt{\frac{\sigma_n}{\sigma_x}}$
If you take the inverse cosine of both sides, you can isolate $\theta$. You are NOT multiplying by inverse cosine to get this.

There are many values of $\theta$ that satisfy the last equation above. One that I get is around 37.6°. Please show us the calculation you did.

4. Oct 1, 2015

### Staff: Mentor

Remember the notation $cos^{-1}(\theta)$ is not the same as $1/cos(\theta)$.

5. Oct 1, 2015

### Mechaman

Thanks for helping I worked it out as the same 37.56 degrees, my maths is a little rusty at the moment!

6. Oct 1, 2015

### Staff: Mentor

Keep in mind that +/-. There's another value around 142°.