Right triangle: A triangle in which one angle is a right angle 
Trigonometric functions on a circle with radius r:
For any angle 
in standard position, let 
be the point on the terminal side of 
that is also on the circle 
.
,
, 
, 
, 
, and 
.
The trigonometric functions of an acute angle:
If 
is an acute angle of a right triangle, the six trigonometric functions of 
are as follow:
, 
, 
, 
, 
, and 
.
Complementary angles:
Two angles are called complementary if their sum equals to 90 degrees.
Angle of elevation:
The acute angle measured from the horizontal to a line-of-sight observation of the object.
Angle of depression:
The acute angle made by the line-of –sight observation of the object and the horizontal.
The trigonometric functions of any angle:
Let 
be an angle in standard position and let P(x,y) be a point, not the origin, on the terminal side that can be in any quadrant. If 
is the distance from the origin to the point P(x,y), then
, 
, 
, 
, 
, and 
Trigonometric identities:
, 
, 
, 
and 
.
