Cosine
The cosine of an angle is the trigonometric ratio of the adjacent side to the hypotenuse of a right triangle containing that angle.
Example:
In the triangle shown, 
and 
.
The cosine ratio is the same regardless of the size of the right triangle. So, it is often easiest to consider a right triangle with a hypotenuse of length 1.
The cosine ratio can also be thought of as a function, which takes different values depending on the measure of the angle. You can measure an angle in degrees or radians.
Suppose you have an angle measuring θ radians. Draw a unit circle on the coordinate plane and draw the angle centered at the origin, with one side as the positive x-axis. Thes-coordinate of the point where the other side of the angle intersects the circle is cos θ, and the y-coordinate is sin θ.
There are a few cosine values that should be memorized, based on 30°-60°-90° triangles and 45°-45°-90° triangles.
Once you know these values, you can derive many other values for the cosine function. Remember that cos θ is positive in quadrants I and II and negative in quadrants III and IV.
To read about graphing these values and seeing the graph of y = cos x, click here.
