Find cos θ given that cos 2θ = 5/6 and 0 ≤ θ < π/2. give an exact answer

now, bear in mind, the square root gives us +/- versions, so, which is it? well, we know the angle is in the range of "0 ≤ θ < π/2", that simply means the 1st quadrant, so, we'll use the positive one then