"We suppose that the wheel rolls to the right.Pbeing at the origin when the turn angletequals 0. The figure shows the wheel after it has turnedtradians. The base of the wheel is at distanceatfrom the origin. The wheel's center is at (at,a), and the coordinates of P are

To express in terms oft,we absorve that for some integer k, so

Thus,

I get everything up to the prior line. How does that equal -sint?