First, realize that in Quandrant I sin and cos are positive
in Quandrant II sin is positive and cos is negative
Using the unit circle pi/4 sin = opp/hyp = 1/sqrt2 = sqrt2/2 (this is the ' y' coordinate)
cos = adj/hyp = 1/sqrt2 = sqrt 2/2 (this is the 'x' coordinate)
(in Quandrant I they are BOTH POSITIVE)
Now move to Quandrant II the magnitude of sin and cos are the same BUT cos is negative
so for 3pi/4 sin = sqrt2/2 cos = - sqrt2/2
so (x,y) on the unit circle for 3pi/4 = ( - sqrt2/2 , sqrt2/2 )