Learning Outcomes
- Convert angle measures between degrees and radians
- Recognize the triangular and circular definitions of the basic trigonometric functions
- Write the basic trigonometric identities
Radian Measure
To use trigonometric functions, we first must understand how to measure the angles. Although we can use both radians and degrees, radians are a more natural measurement because they are related directly to the unit circle, a circle with radius 1. The radian measure of an angle is defined as follows. Given an angle [latex]\theta[/latex], let [latex]s[/latex] be the length of the corresponding arc on the unit circle (Figure 1). We say the angle corresponding to the arc of length 1 has radian measure 1.
Since an angle of 360° corresponds to the circumference of a circle, or an arc of length [latex]2\pi[/latex], we conclude that an angle with a degree measure of 360° has a radian measure of [latex]2\pi[/latex]. Similarly, we see that 180° is equivalent to [latex]\pi[/latex] radians. The table below shows the relationship between common degree and radian values.
Degrees | Radians | Degrees | Radians |
---|---|---|---|
0 | 0 | 120 | [latex]2\pi/3[/latex] |
30 | [latex]\pi/6[/latex] | 135 | [latex]3\pi/4[/latex] |
45 | [latex]\pi/4[/latex] | 150 | [latex]5\pi/6[/latex] |
60 | [latex]\pi/3[/latex] | 180 | [latex]\pi[/latex] |
90 | [latex]\pi/2[/latex] |
Recall: radian and degree conversions
Converting between Radians and Degrees
Because degrees and radians both measure angles, we need to be able to convert between them. We can easily do so using a proportion.
This proportion shows that the measure of angle [latex]\theta[/latex] in degrees divided by 180 equals the measure of angle [latex]\theta[/latex] in radians divided by [latex]\pi .[/latex] Or, phrased another way, degrees is to 180 as radians is to [latex]\pi[/latex].
Converting between Radians and Degrees
To convert between degrees and radians, use the proportion
Example: Converting between Radians and Degrees
- Express 225° using radians.
- Express [latex]\dfrac{5\pi}{3}[/latex] rad using degrees.
Try It
Express 210° using radians. Express [latex]\dfrac{11\pi}{6}[/latex] rad using degrees.
Try It
The Six Basic Trigonometric Functions
Trigonometric functions allow us to use angle measures, in radians or degrees, to find the coordinates of a point on any circle—not only on a unit circle—or to find an angle given a point on a circle. They also define the relationship among the sides and angles of a triangle.
To define the trigonometric functions, first consider the unit circle centered at the origin and a point [latex]P=(x,y)[/latex] on the unit circle. Let [latex]\theta[/latex] be an angle with an initial side that lies along the positive [latex]x[/latex]-axis and with a terminal side that is the line segment [latex]OP[/latex]. An angle in this position is said to be in standard position (Figure 2). We can then define the values of the six trigonometric functions for [latex]\theta[/latex] in terms of the coordinates [latex]x[/latex] and [latex]y[/latex].
Definition
Let [latex]P=(x,y)[/latex] be a point on the unit circle centered at the origin [latex]O[/latex]. Let [latex]\theta[/latex] be an angle with an initial side along the positive [latex]x[/latex]-axis and a terminal side given by the line segment [latex]OP[/latex]. The trigonometric functions are then defined as
If [latex]x=0[/latex], then [latex]\sec \theta[/latex] and [latex]\tan \theta[/latex] are undefined. If [latex]y=0[/latex], then [latex]\cot \theta[/latex] and [latex]\csc \theta[/latex] are undefined.
We can see that for a point [latex]P=(x,y)[/latex] on a circle of radius [latex]r[/latex] with a corresponding angle [latex]\theta[/latex], the coordinates [latex]x[/latex] and [latex]y[/latex] satisfy
The values of the other trigonometric functions can be expressed in terms of [latex]x, \, y[/latex], and [latex]r[/latex] (Figure 3).
The table below shows the values of sine and cosine at the major angles in the first quadrant. From this table, we can determine the values of sine and cosine at the corresponding angles in the other quadrants. The values of the other trigonometric functions are calculated easily from the values of [latex]\sin \theta[/latex] and [latex]\cos \theta[/latex].
[latex]\theta[/latex] | [latex]\sin \theta[/latex] | [latex]\cos \theta[/latex] |
---|---|---|
0 | 0 | 1 |
[latex]\large{\frac{\pi}{6}}[/latex] | [latex]\large{\frac{1}{2}}[/latex] | [latex]\large{\frac{\sqrt{3}}{2}}[/latex] |
[latex]\large{\frac{\pi}{4}}[/latex] | [latex]\large{\frac{\sqrt{2}}{2}}[/latex] | [latex]\large{\frac{\sqrt{2}}{2}}[/latex] |
[latex]\large{\frac{\pi}{3}}[/latex] | [latex]\large{\frac{\sqrt{3}}{2}}[/latex] | [latex]\large{\frac{1}{2}}[/latex] |
[latex]\large{\frac{\pi}{2}}[/latex] | 1 | 0 |
Example: Evaluating Trigonometric Functions
Evaluate each of the following expressions.
- [latex]\sin \Big(\large\frac{2\pi}{3}\Big)[/latex]
- [latex]\cos \Big(-\large\frac{5\pi}{6}\Big)[/latex]
- [latex]\tan \Big(\large\frac{15\pi}{4}\Big)[/latex]
Try It
Evaluate [latex]\cos \left(\dfrac{3\pi}{4}\right)[/latex] and [latex]\sin \left(\dfrac{−\pi}{6}\right)[/latex].
Try It
As mentioned earlier, the ratios of the side lengths of a right triangle can be expressed in terms of the trigonometric functions evaluated at either of the acute angles of the triangle. Let [latex]\theta[/latex] be one of the acute angles. Let [latex]A[/latex] be the length of the adjacent leg, [latex]O[/latex] be the length of the opposite leg, and [latex]H[/latex] be the length of the hypotenuse. By inscribing the triangle into a circle of radius [latex]H[/latex], as shown in Figure 7, we see that [latex]A, \, H[/latex], and [latex]O[/latex] satisfy the following relationships with [latex]\theta[/latex]:
Example: Constructing a Wooden Ramp
A wooden ramp is to be built with one end on the ground and the other end at the top of a short staircase. If the top of the staircase is 4 ft from the ground and the angle between the ground and the ramp is to be [latex]10^{\circ}[/latex], how long does the ramp need to be?
Watch the following video to see the worked solution to Example: Constructing a Wooden Ramp
Try It
A house painter wants to lean a 20-ft ladder against a house. If the angle between the base of the ladder and the ground is to be [latex]60^{\circ}[/latex], how far from the house should she place the base of the ladder?
Trigonometric Identities
A trigonometric identity is an equation involving trigonometric functions that is true for all angles [latex]\theta[/latex] for which the functions are defined. We can use the identities to help us solve or simplify equations. The main trigonometric identities are listed next.
Trigonometric Identities
Reciprocal identities
Pythagorean identities
Addition and subtraction formulas
Double-angle formulas
Recall: Given a trigonometric equation, solve using algebra.
- Look for a pattern that suggests an algebraic property, such as the difference of squares or a factoring opportunity.
- Substitute the trigonometric expression with a single variable, such as [latex]x[/latex] or [latex]u[/latex].
- Solve the equation the same way an algebraic equation would be solved.
- Substitute the trigonometric expression back in for the variable in the resulting expressions.
- Solve for the angle.
Try It
Additionally, trigonometric equations will have an infinite number of solutions. If we need to find all possible solutions, then we must add [latex]2\pi k[/latex], where [latex]k[/latex] is an integer, to the initial solution. Recall the rule that gives the format for stating all possible solutions for a function where the period is [latex]2\pi :[/latex]
Example: Solving Trigonometric Equations
For each of the following equations, use a trigonometric identity to find all solutions.
-
- [latex]1+\cos(2\theta)=\cos \theta[/latex]
- [latex]\sin(2\theta)=\tan \theta[/latex]
Watch the following video to see the worked solution to Example: Solving Trigonometric Equations
Try It
Find all solutions to the equation [latex]\cos(2\theta)=\sin \theta[/latex].
Trigonometric identities are not only useful for solving trigonometric equations, they can also help us verify identities.
Recall: Given a trigonometric identity, verify that it is true.
- Work on one side of the equation. It is usually better to start with the more complex side, as it is easier to simplify than to build.
- Look for opportunities to factor expressions, square a binomial, or add fractions.
- Noting which functions are in the final expression, look for opportunities to use the identities and make the proper substitutions.
- If these steps do not yield the desired result, try converting all terms to sines and cosines.
Example: Proving a Trigonometric Identity
Prove the trigonometric identity [latex]1+\tan^2 \theta =\sec^2 \theta[/latex].
Try It
Prove the trigonometric identity [latex]1+\cot^2 \theta =\csc^2 \theta[/latex].
Candela Citations
- 1.3 Trigonometric Functions. Authored by: Ryan Melton. License: CC BY: Attribution
- 1.3.3. Authored by: Ryan Melton. License: CC BY: Attribution
- Calculus Volume 1. Authored by: Gilbert Strang, Edwin (Jed) Herman. Provided by: OpenStax. Located at: https://openstax.org/details/books/calculus-volume-1. License: CC BY-NC-SA: Attribution-NonCommercial-ShareAlike. License Terms: Access for free at https://openstax.org/books/calculus-volume-1/pages/1-introduction
[latex]\pi[/latex] radians is equal to [latex]180^{\circ}[/latex].