Mr. Unit Circle.

The unit circle has a radius of 1. Because the radius is one, we can measure sine,cosine, and tangent. The Pythagoras theorem is a^2+b^2=c^2, it only works for right triangles.  When we solve trig problems, unit circle is extremely helpful. It saves up a lot of time. Triangles constructed on the unit circle can also be used to illustrate the periodicity of the trigonometric functions. The interior of the unit circle is called the open unit disk, while the interior of the unit circle combined with the unit circle itself is called the closed unit disk. I like unit circles because they make things easy.