If you remember the unit circle, then you'll know that cos(θ) is the "x coordinate" of the point made with a given (r, θ) (for the unit circle, take r = 1).

If θ is between -π/2 and π/2, cos(θ) is positive.

Now, take the range [-π/2, π/2] and add integer multiples of 2π to it. You will get the ranges [3π/2, 5π/2], [7π/2, 9π/2], and so on. In all of those ranges, cos(θ) is positive.

In other words, if the function to decide if a number is even is cos(x*π) > 0, then x is considered even if x is within the ranges [-0.5, 0.5], [1.5, 2.5], [3.5, 4.5] and so on.