Solving for cos θ gives us:

If , then the reference angle is α = 1.3181.

So for , we have θ in the first and 4th quadrants. So

θ = 1.3181 or 4.9651

For , we have θ in the 2nd and 3rd quadrants. So

θ = 1.8235 or 4.4597

So θ= 1.3181, 1.8235, 4.4597 or 4.9651 radians.

We can see from the graph of that our answer is correct: