<< Chapter < Page Chapter >> Page >

Using reference angles to find sine and cosine

  1. Using a reference angle, find the exact value of cos ( 150° ) and sin ( 150° ) .
  2. Using the reference angle, find cos 5 π 4 and sin 5 π 4 .
  1. 150° is located in the second quadrant. The angle it makes with the x -axis is 180° 150° = 30° , so the reference angle is 30° .

    This tells us that 150° has the same sine and cosine values as 30° , except for the sign.

    cos ( 30° ) = 3 2 and sin ( 30° ) = 1 2

    Since 150° is in the second quadrant, the x -coordinate of the point on the circle is negative, so the cosine value is negative. The y -coordinate is positive, so the sine value is positive.

    cos ( 150° ) = 3 2 and sin ( 150° ) = 1 2
  2. 5 π 4 is in the third quadrant. Its reference angle is 5 π 4 π = π 4 . The cosine and sine of π 4 are both 2 2 . In the third quadrant, both x and y are negative, so:
    cos 5 π 4 = 2 2 and sin 5 π 4 = 2 2
Got questions? Get instant answers now!
Got questions? Get instant answers now!
  1. Use the reference angle of 315° to find cos ( 315° ) and sin ( 315° ) .
  2. Use the reference angle of π 6 to find cos ( π 6 ) and sin ( π 6 ) .
  1. cos ( 315° ) = 2 2 ,   sin ( 315° ) = 2 2
  2. cos ( π 6 ) = 3 2 , sin ( π 6 ) = 1 2
Got questions? Get instant answers now!

Using reference angles to find coordinates

Now that we have learned how to find the cosine and sine values for special angles in the first quadrant, we can use symmetry and reference angles to fill in cosine and sine values for the rest of the special angles on the unit circle    . They are shown in [link] . Take time to learn the ( x , y ) coordinates of all of the major angles in the first quadrant.

Graph of unit circle with angles in degrees, angles in radians, and points along the circle inscribed.
Special angles and coordinates of corresponding points on the unit circle

In addition to learning the values for special angles, we can use reference angles to find ( x , y ) coordinates of any point on the unit circle, using what we know of reference angles along with the identities

x = cos  t y = sin  t

First we find the reference angle corresponding to the given angle. Then we take the sine and cosine values of the reference angle, and give them the signs corresponding to the y - and x -values of the quadrant.

Given the angle of a point on a circle and the radius of the circle, find the ( x , y ) coordinates of the point.

  1. Find the reference angle by measuring the smallest angle to the x -axis.
  2. Find the cosine and sine of the reference angle.
  3. Determine the appropriate signs for x and y in the given quadrant.

Using the unit circle to find coordinates

Find the coordinates of the point on the unit circle at an angle of 7 π 6 .

We know that the angle 7 π 6 is in the third quadrant.

First, let’s find the reference angle by measuring the angle to the x -axis. To find the reference angle of an angle whose terminal side is in quadrant III, we find the difference of the angle and π .

7 π 6 π = π 6

Next, we will find the cosine and sine of the reference angle.

cos ( π 6 ) = 3 2 sin ( π 6 ) = 1 2

We must determine the appropriate signs for x and y in the given quadrant. Because our original angle is in the third quadrant, where both x and y are negative, both cosine and sine are negative.

cos ( 7 π 6 ) = 3 2 sin ( 7 π ) = 1 2

Now we can calculate the ( x , y ) coordinates using the identities x = cos θ and y = sin θ .

The coordinates of the point are ( 3 2 , 1 2 ) on the unit circle.

Got questions? Get instant answers now!
Got questions? Get instant answers now!

Find the coordinates of the point on the unit circle at an angle of 5 π 3 .

( 1 2 , 3 2 )

Got questions? Get instant answers now!

Questions & Answers

what does preconceived mean
sammie Reply
physiological Psychology
Nwosu Reply
How can I develope my cognitive domain
Amanyire Reply
why is communication effective
Dakolo Reply
Communication is effective because it allows individuals to share ideas, thoughts, and information with others.
effective communication can lead to improved outcomes in various settings, including personal relationships, business environments, and educational settings. By communicating effectively, individuals can negotiate effectively, solve problems collaboratively, and work towards common goals.
it starts up serve and return practice/assessments.it helps find voice talking therapy also assessments through relaxed conversation.
miss
Every time someone flushes a toilet in the apartment building, the person begins to jumb back automatically after hearing the flush, before the water temperature changes. Identify the types of learning, if it is classical conditioning identify the NS, UCS, CS and CR. If it is operant conditioning, identify the type of consequence positive reinforcement, negative reinforcement or punishment
Wekolamo Reply
please i need answer
Wekolamo
because it helps many people around the world to understand how to interact with other people and understand them well, for example at work (job).
Manix Reply
Agreed 👍 There are many parts of our brains and behaviors, we really need to get to know. Blessings for everyone and happy Sunday!
ARC
A child is a member of community not society elucidate ?
JESSY Reply
Isn't practices worldwide, be it psychology, be it science. isn't much just a false belief of control over something the mind cannot truly comprehend?
Simon Reply
compare and contrast skinner's perspective on personality development on freud
namakula Reply
Skinner skipped the whole unconscious phenomenon and rather emphasized on classical conditioning
war
explain how nature and nurture affect the development and later the productivity of an individual.
Amesalu Reply
nature is an hereditary factor while nurture is an environmental factor which constitute an individual personality. so if an individual's parent has a deviant behavior and was also brought up in an deviant environment, observation of the behavior and the inborn trait we make the individual deviant.
Samuel
I am taking this course because I am hoping that I could somehow learn more about my chosen field of interest and due to the fact that being a PsyD really ignites my passion as an individual the more I hope to learn about developing and literally explore the complexity of my critical thinking skills
Zyryn Reply
good👍
Jonathan
and having a good philosophy of the world is like a sandwich and a peanut butter 👍
Jonathan
generally amnesi how long yrs memory loss
Kelu Reply
interpersonal relationships
Abdulfatai Reply
What would be the best educational aid(s) for gifted kids/savants?
Heidi Reply
treat them normal, if they want help then give them. that will make everyone happy
Saurabh
What are the treatment for autism?
Magret Reply
hello. autism is a umbrella term. autistic kids have different disorder overlapping. for example. a kid may show symptoms of ADHD and also learning disabilities. before treatment please make sure the kid doesn't have physical disabilities like hearing..vision..speech problem. sometimes these
Jharna
continue.. sometimes due to these physical problems..the diagnosis may be misdiagnosed. treatment for autism. well it depends on the severity. since autistic kids have problems in communicating and adopting to the environment.. it's best to expose the child in situations where the child
Jharna
child interact with other kids under doc supervision. play therapy. speech therapy. Engaging in different activities that activate most parts of the brain.. like drawing..painting. matching color board game. string and beads game. the more you interact with the child the more effective
Jharna
results you'll get.. please consult a therapist to know what suits best on your child. and last as a parent. I know sometimes it's overwhelming to guide a special kid. but trust the process and be strong and patient as a parent.
Jharna
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply
Practice Key Terms 3

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Algebra and trigonometry. OpenStax CNX. Nov 14, 2016 Download for free at https://legacy.cnx.org/content/col11758/1.6
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Algebra and trigonometry' conversation and receive update notifications?

Ask