On Jan 19, 11:44 pm, Albert <albert.xtheunkno...@gmail.com> wrote: > On Jan 20, 10:25 am, Matt <matt271829-n...@yahoo.co.uk> wrote: > > > You've seen that cos(angle) = x/r > <snip> > > If the angle is 90 degrees then what is the > > corresponding value of x? > > Now I see: x is zero and therefore cos90 = 0.
Possibly I'm just repeating what's already been said, but imagine a ray of length 1 that has one end fixed at the origin, and the other end sweeping out a circle of radius 1. The x-coordinate of the free end is cos(A), and the y-coordinate is sin(A), where A is the angle the ray makes with the positive x-axis.
Anticlockwise rotation from the positive x-axis means a positive angle; clockwise rotation means a negative angle. Once you've gone around a full circle (2*pi radians, 360 degrees) you're back where you started, so sine and cosine are periodic with period 2*pi.