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In this section you will:
  • Find function values for the sine and cosine of 30°  or  ( π 6 ) , 45°  or  ( π 4 ) , and 60  or  ( π 3 ) .
  • Identify the domain and range of sine and cosine functions.
  • Find reference angles.
  • Use reference angles to evaluate trigonometric functions.
Photo of a ferris wheel.
The Singapore Flyer is the world’s tallest Ferris wheel. (credit: ʺVibin JKʺ/Flickr)

Looking for a thrill? Then consider a ride on the Singapore Flyer, the world’s tallest Ferris wheel. Located in Singapore, the Ferris wheel soars to a height of 541 feet—a little more than a tenth of a mile! Described as an observation wheel, riders enjoy spectacular views as they travel from the ground to the peak and down again in a repeating pattern. In this section, we will examine this type of revolving motion around a circle. To do so, we need to define the type of circle first, and then place that circle on a coordinate system. Then we can discuss circular motion in terms of the coordinate pairs.

Finding trigonometric functions using the unit circle

We have already defined the trigonometric functions in terms of right triangles. In this section, we will redefine them in terms of the unit circle. Recall this a unit circle is a circle centered at the origin with radius 1, as shown in [link] . The angle (in radians) that t intercepts forms an arc of length s . Using the formula s = r t , and knowing that r = 1 , we see that for a unit circle, s = t .

The x- and y- axes divide the coordinate plane into four quarters called quadrants. We label these quadrants to mimic the direction a positive angle would sweep. The four quadrants are labeled I, II, III, and IV.

For any angle t , we can label the intersection of the terminal side and the unit circle as by its coordinates, ( x , y ) . The coordinates x and y will be the outputs of the trigonometric functions f ( t ) = cos t and f ( t ) = sin t , respectively. This means x = cos  t and y = sin  t .

Graph of a circle with angle t, radius of 1, and an arc created by the angle with length s. The terminal side of the angle intersects the circle at the point (x,y).
Unit circle where the central angle is t radians

Unit circle

A unit circle    has a center at ( 0 , 0 ) and radius 1. In a unit circle, the length of the intercepted arc is equal to the radian measure of the central angle t .

Let ( x , y ) be the endpoint on the unit circle of an arc of arc length     s . The ( x , y ) coordinates of this point can be described as functions of the angle.

Defining sine and cosine functions from the unit circle

The sine function relates a real number t to the y -coordinate of the point where the corresponding angle intercepts the unit circle. More precisely, the sine of an angle t equals the y -value of the endpoint on the unit circle of an arc of length t . In [link] , the sine is equal to y . Like all functions, the sine function    has an input and an output. Its input is the measure of the angle; its output is the y -coordinate of the corresponding point on the unit circle.

The cosine function    of an angle t equals the x -value of the endpoint on the unit circle of an arc of length t . In [link] , the cosine is equal to x .

Illustration of an angle t, with terminal side length equal to 1, and an arc created by angle with length t. The terminal side of the angle intersects the circle at the point (x,y), which is equivalent to (cos t, sin t).

Because it is understood that sine and cosine are functions, we do not always need to write them with parentheses: sin t is the same as sin ( t ) and cos t is the same as cos ( t ) . Likewise, cos 2 t is a commonly used shorthand notation for ( cos ( t ) ) 2 . Be aware that many calculators and computers do not recognize the shorthand notation. When in doubt, use the extra parentheses when entering calculations into a calculator or computer.

Questions & Answers

find to nearest one decimal place of centimeter the length of an arc of circle of radius length 12.5cm and subtending of centeral angle 1.6rad
Martina Reply
factoring polynomial
Noven Reply
what's your topic about?
Shin Reply
find general solution of the Tanx=-1/root3,secx=2/root3
Nani Reply
find general solution of the following equation
the value of 2 sin square 60 Cos 60
Sanjay Reply
when can I use sin, cos tan in a giving question
duru Reply
depending on the question
I am a carpenter and I have to cut and assemble a conventional roof line for a new home. The dimensions are: width 30'6" length 40'6". I want a 6 and 12 pitch. The roof is a full hip construction. Give me the L,W and height of rafters for the hip, hip jacks also the length of common jacks.
I want to learn the calculations
Koru Reply
where can I get indices
Kojo Reply
I need matrices
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maybe provide us videos
about complex fraction
What do you mean by a
nothing. I accidentally press it
you guys know any app with matrices?
Solve the x? x=18+(24-3)=72
Leizel Reply
x-39=72 x=111
Solve the formula for the indicated variable P=b+4a+2c, for b
Deadra Reply
Need help with this question please
b= p - 4a - 2c
like Deadra, show me the step by step order of operation to alive for b
A laser rangefinder is locked on a comet approaching Earth. The distance g(x), in kilometers, of the comet after x days, for x in the interval 0 to 30 days, is given by g(x)=250,000csc(π30x). Graph g(x) on the interval [0, 35]. Evaluate g(5)  and interpret the information. What is the minimum distance between the comet and Earth? When does this occur? To which constant in the equation does this correspond? Find and discuss the meaning of any vertical asymptotes.
Kaitlyn Reply
The sequence is {1,-1,1-1.....} has
amit Reply
circular region of radious
Kainat Reply
how can we solve this problem
Joel Reply
Sin(A+B) = sinBcosA+cosBsinA
Eseka Reply
Prove it
Please prove it
Practice Key Terms 3

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Source:  OpenStax, Algebra and trigonometry. OpenStax CNX. Nov 14, 2016 Download for free at https://legacy.cnx.org/content/col11758/1.6
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