/sci/ - Science & Math

Can someone help me with this? Not really sure what to do here. For a, I assume the average should be 0 mainly because it's a sinusoidal wave, but I don't know how the k values and probability come in to play.
I also know that the variance is [x^2] - average^2, so I guess just x^2 if the average is 0. Don't really know how to attempt this, though.