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4.2
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Trigonometric Functions:
The Unit Circle
Identify the unit circle and describe its
relationship to real numbers.
Evaluate trigonometric functions using the unit
circle.
Use domain and period to evaluate sine and
cosine functions and use a calculator to
evaluate trigonometric functions.
Copyright © Cengage Learning. All rights reserved.
The Unit Circle
The two historical perspectives of trigonometry incorporate different methods of
introducing the trigonometric functions.
One such perspective is based on the unit circle.
Consider the unit circle given by
as shown in figure.
x2 + y2 = 1
Imagine wrapping the real number line around this circle, with positive numbers
corresponding to a counterclockwise wrapping and negative numbers
corresponding to a clockwise wrapping, as shown below.
2
The Unit Circle
As the real number line is wraps around the unit circle, each real number t
corresponds to a point
(x, y)
on the circle.
For example, the real number 0 corresponds to the point (1, 0). Moreover, because
the unit circle has a circumference of 2, the real number 2 also corresponds to
the point (1, 0).
In general, each real number t also corresponds to a central angle  (in standard
position) whose radian measure is t. With this interpretation of t, the arc length
formula
s = r
(with r = 1) indicates that the real number t is the (directional) length of the arc
intercepted by the angle , given in radians.
3
The Trigonometric Functions
The coordinates x and y are two functions of the real variable t. You can use these
coordinates to define the six trigonometric functions of t.
sine
cosecant
cosine
secant
tangent
cotangent
These six functions are normally abbreviated sin, cos, tan, csc, sec, and cot,
respectively.
4
The Trigonometric Functions
In the definitions of the trigonometric functions, note that the tangent and secant
are not defined when x = 0.
For instance, because t =  /2 corresponds to (x, y) = (0, 1), it follows that tan( /2)
undefined
and sec( /2) are ___________.
Similarly, the cotangent and cosecant are not defined when y = 0.
For instance, because t = 0 corresponds to (x, y) = (1, 0), cot 0 and csc 0 are
undefined.
Example 1: Evaluate the six trigonometric functions at each real number.
a.
5
Evaluating Trigonometric Functions
b.
6
Evaluating Trig. Functions
c.
t=
cont’d
is the point (–1, 0).
d.
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Domain and Period of Sine and Cosine
The domain of the sine and cosine functions is the set of all real numbers.
To determine the range of these two functions, consider the unit circle shown in the
figure.
By definition, sin t = y and cos t = x. Because (x, y) is on the unit circle, you know
that −1  y  1 and −1  x  1. So, the values of sine and cosine also range
between −1 and 1.
So, the values of sine and cosine also range between –1 and 1.
–1  y  1
–1  sin t  1
–1  x  1
and
–1  cos t  1
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Domain and Period of Sine and Cosine
Adding 2 to each value of t in the interval [0, 2] completes a second revolution
around the unit circle, as shown in the figure below.
The values of sin(t + 2) and cos(t + 2) correspond to those of sin t and cos t.
Similar results can be obtained for repeated revolutions (positive or negative)
around the unit circle. This leads to the general result
sin(t + 2 n) = sin t
and
cos(t + 2 n) = cos t
for any integer n and real number t.
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Domain and Period of Sine and Cosine
periodic
Functions that behave in such a repetitive (or cyclic) manner are called __________.
It follows from the definition of periodic function that the sine and cosine functions
are periodic and have a period of 2. The other four trigonometric functions are
also periodic.
A function f is even when
f(–t) = f(t)
and is odd when
f(–t) = –f(t)
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Example 2 – Using the Period to Evaluate Sine and Cosine
, because the function is odd.
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Evaluate Trig. Functions
cont’d
** Make sure your calculator is radian mode!
0.9238795325
Must use reciprocal of sin, when putting it
in to the calculator.
7.086167396
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