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 9.1 Basic Functions 9.2 Cosecant, Secant, and Cotangent 9.3 Solving Right Triangles 9.4 Trigonometric Identities 9.5 Graphing Trigonometric Functions 9.6 The Unit Circle

 9.7 Graphing in the Entire Coordinate Plane 9.8 Inverse Trigonometric Functions 9.9 Solving Non-Right Triangles 9.10 Key Terms 9.11 Review Questions 9.12 Explanations
The Unit Circle
The unit circle is a circle whose center is the origin and whose radius is 1. It is defined by the equation x2 + y2 = 1.
The most useful and interesting property of the unit circle is that the coordinates of a given point on the circle can be found using only the knowledge of the measure of the angle.
Any radius of the unit circle is the hypotenuse of a right triangle that has a (horizontal) leg of length cos and a (vertical) leg of length sin . The angle is defined as the radius measured in standard position. These relationships are easy to see using the trigonometric functions:
As you can see, because the radius of the unit circle is 1, the trigonometric functions sine and cosine are simplified: sin = y and cos = x. This means that another way to write the coordinates of a point (x, y) on the unit circle is (cos , sin ), where is the measure of the angle in standard position whose terminal side contains the point.
Here’s an example of a typical question on the Math IIC that tests this principle:
 What are the coordinates of the point P pictured below?
Point P is the endpoint of a radius of the unit circle that forms a 30º angle with the negative x-axis. This means that an angle of 210º in standard position would terminate in the same position. So, the coordinates of the point are (cos 210º, sin 210º) = (–/2, –1/2). Both coordinates must be negative, since the point is in the third quadrant.
Range
The unit circle also provides a lot of information about the range of trigonometric functions and the values of the functions at certain angles.
For example, because the unit circle has a radius of one and has points all of the form (cos , sin ), we know that:
Tangent ranges from –∞ to ∞, but is undefined at every angle whose cosine is 0. Can you guess why? Look at the formula of tan = sin / cos . If cos = 0, then division by 0 occurs, and the quotient, tan , is undefined.