The reference angle 
of a nonquadrantal angle 
in angle standard
position is the acute angle between the terminal
side of 
and the x-axis. It is given by
 |
so 
.
Angles whose terminal sides
lie on the x-axis or y-axis
have no reference angle under this standard convention. Coterminal
angles have the same reference angle. Reference angles reduce the evaluation
of trigonometric functions in any quadrant
to the corresponding values in the first quadrant, together
with the appropriate sign.
