# Thread: Angle of rotation in a matrix

1. ## Angle of rotation in a matrix

The matrix [-0.8 -0.6
0.6 -0.8] (a 2x2 matrix)

represents a rotation. Find the angle of rotation (in radians).

This one has me stumped, any help would be appreciated. The answer is 2.5 radians but the approach is where I am lost.

2. You are probably expected to know that a rotation $\theta$ can be represented by the matrix
$\left[\begin{array}{cc} \cos(\theta) & -\sin(\theta) \\ \sin(\theta) & \cos(\theta) \end{array}\right]$
This does not, however, give 2.5 radians so I suspect you have either copied this out wrong or the source is incorrect.

3. I understand that is the rotation matrix, what I don't understand is how you come up with the final angle of rotation. Do you plug in the numbers into the corresponding thetas? Thanks again for the help. I would be more specific but I have to head to work. Any help would be appreciated.

You are probably expected to know that a rotation $\theta$ can be represented by the matrix
$\left[\begin{array}{cc} \cos(\theta) & -\sin(\theta) \\ \sin(\theta) & \cos(\theta) \end{array}\right]$
This does not, however, give 2.5 radians so I suspect you have either copied this out wrong or the source is incorrect.
I think it does give 2.5 radians (to 2 significant figures). If $\cos\theta$ is negative and $\sin\theta$ is positive then $\pi/2<\theta<\pi$. The angle in that interval with $\cos\theta = -0.8$ is $\pi - \arccos0.8 \approx 3.14 - 0.64 = 2.5$.

5. Opalg is right, sorry to have confused you