If 
is an acute angle, then we can draw
a right triangle, as in (A) of
the figure below. The side
opposite the angle is labeled b, the side
adjacent to is labeled a and the hypotenuse
is labeled c. In (B), we have placed
the vertex A at the origin, and the adjacent side on the x-axis.
The vertex B is at the point (a,b).
acute.epsAlternate definitions for 
and 
are:
Note that when c=1, the point B(a,b) lies on the
unit circle, and 
is
the second coordinate of the point B(a,b), while 
is the first coordinate of this point.
Thus these alternate definitions are consistent with the old
definitions of sine and cosine.