1.22: Domain, Range, and Signs of Trigonometric Functions
You are doodling in art class one day when you draw a circle. Then you draw a few lines extending outward from the center to the edge of the circle. You draw a triangle with the "x" axis, and realize that you're thinking about your math class again.
You notice that the relationship for the sine function involves the length of the side opposite the angle divided by the length of the hypotenuse. But while the hypotenuse is always a positive number, the sign of the opposite side can be different, depending on what quadrant the angle is drawn in.
Can you determine what the sign of the sine function will be in each of the four quadrants, based on the knowledge of the ratio that defines the sine function?
Watch This
James Sousa: Domain, Range, and Signs of Trigonometric Functions
Guidance
While the trigonometric functions may seem quite different from other functions you have worked with, they are in fact just like any other function. We can think of a trig function in terms of “input” and “output.” The input is always an angle. The output is a ratio of sides of a triangle. If you think about the trig functions in this way, you can define the domain and range of each function.
Let’s first consider the sine and cosine functions. The input of each of these functions is always an angle, and as you learned in the previous sections, these angles can take on any real number value. Therefore the sine and cosine function have the same domain, the set of all real numbers,
In this right triangle,
In either case, the minimum value is 1. For example,
The table below summarizes the domains and ranges of these functions:
Domain  Range  

Sine 


Cosine 


Knowing the domain and range of the cosine and sine function can help us determine the domain and range of the secant and cosecant function. First consider the sine and cosecant functions, which as we showed above, are reciprocals. The cosecant function will be defined as long as the sine value is not 0. Therefore the domain of the cosecant function excludes all angles with sine value 0, which are
In Chapter 2 you will analyze the graphs of these functions, which will help you see why the reciprocal relationship results in a particular range for the cosecant function. Here we will state this range, and in the review questions you will explore values of the sine and cosecant function in order to begin to verify this range, as well as the domain and range of the secant function.
Domain  Range  

Cosecant 


Secant 


Now let’s consider the tangent and cotangent functions. The tangent function is defined as
Function  Domain  Range 

Tangent 

All reals 
Cotangent 

All reals 
Knowing the ranges of these functions tells you the values you should expect when you determine the value of a trig function of an angle. However, for many problems you will need to identify the sign of the function of an angle: Is it positive or negative?
In determining the ranges of the sine and cosine functions above, we began to categorize the signs of these functions in terms of the quadrants in which angles lie. The figure below summarizes the signs for angles in all 4 quadrants.
An easy way to remember this is “All Students Take Calculus.” Quadrant I: All values are positive, Quadrant II: Sine is positive, Quadrant III: Tangent is positive, and Quadrant IV: Cosine is positive. This simple memory device will help you remember which trig functions are positive and where.
Example A
State the sign of
Solution: The angle
Example B
State the sign of
Solution: The angle
Example C
State the sign of
Solution: The angle
Vocabulary
Domain: The domain of a function is the set of 'x' values for which the function is defined.
Range: The range of a function is the set of 'y' values for which the function is defined.
Guided Practice
1. State the sign of
2. State the sign of
3. State the sign of
Solutions:
1. The angle
2. The angle
3. The angle
Concept Problem Solution
Since the sine function is defined to be the length of the opposite side divided by the length of the hypotenuse, the sign of the sine function is the sign of the "y" coordinate for whatever quadrant is being considered. In quadrants 1 and 2, the "y" coordinate is positive, so the sine function is positive. In quadrants 3 and 4, the "y" coordinate is negative, so the sine function is negative as well.
Practice
 In what quadrants is the sine function positive?
 In what quadrants is the cotangent function negative?
 In what quadrants is the cosine function negative?
 In what quadrants is the tangent function positive?
 For what angles is the cosecant function undefined?
 State the sign of
sin510∘ .  State the sign of
cos315∘ .  State the sign of
tan135∘ .  State the sign of
cot220∘ .  State the sign of
csc40∘ .  State the sign of
cos330∘ .  State the sign of
sin60∘ .  State the sign of
sec−45∘ .  Explain why the cosecant function is never equal to 0.
 Using your knowledge of domain and range, make a possible sketch for
y=sinx .
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Image Attributions
Here you'll learn the domain and range, as well as the sign in different quadrants, for six trig functions.