Radian and Degree Measure

1. 6-1 & 6-2 Radian and Degree Measure

2. Objectives: Describe angles Use radian measure Use degree measure Use angles to model and solve real-life problems

3. Angles

4. Angles As derived from the Greek language, the word trigonometry means “measurement of triangles.” Initially, trigonometry dealt with relationships among the sides and angles of triangles. An angle is determined by rotating a ray (half-line) about its endpoint.

5. Angles The starting position of the ray is the initial side of the angle, and the position after rotation is the terminal side, as shown. Angle

6. Angles The endpoint of the ray is the vertex of the angle. This perception of an angle fits a coordinate system in which the origin is the vertex and the initial side coincides with the positive x-axis. Such an angle is in standard position, as shown. Angle Angle in standard position

7. Angles Counterclockwise rotation generates positive angles and clockwise rotation generates negative angles. Angles are labeled with Greek letters such as  (alpha),  (beta), and  (theta), as well as uppercase letters A, B, and C. Note that angles  and  have the same initial andterminal sides. Such angles are coterminal. Coterminal angles

8. 120° –210° Angle describes the amount and direction of rotation Positive Angle- rotates counter-clockwise (CCW) Negative Angle- rotates clockwise (CW)

9. Angles - Review Angle Angle = determined by rotating a ray (half-line) about its endpoint. Initial Side = the starting point of the ray Terminal Side = the position after rotation Vertex = the endpoint of the ray Positive Angles = generated by counterclockwise rotation Negative Angles = generated by clockwise rotation Angle in standard position

10. Radian Measure

11. Radian Measure Central Angle Measure of an Angle = determined by the amount of rotation from the initial side to the terminal side (one way to measure angle is in radians) Central Angles = an angle whose vertex is the center of the circle

12. Radian Measure You determine themeasure of an angle by the amount of rotation from the initial side to the terminal side. One way to measure angles is in radians. This type of measure is especially useful in calculus. To define a radian, you can use a central angle of a circle, one whose vertex is the center of the circle, as shown. Arc length = radius when  =1 radian

13. Radian Measure Given a circle of radius (r)with the vertex of an angle as the center of the circle, if the arc length (s) formed by intercepting the circle with the sides of the angle is the same length as the radius (r), the angle measures one radian. terminal side arc length is also r (s=r) s r r initial side This angle measures 1 radian radius of circle is r

14. Radian Measure • The radian measure of a central angle θ is obtained by dividing the arc length s by r (θ = s/r). • Because the circumference of a circle is 2r units, it follows that a central angle of one full revolution (counterclockwise) corresponds to an arc length of s = 2r. • Therefore, a full circle measures , or 2𝜋 radians.

15. Radian Measure Moreover, because 26.28, there are just over six radius lengths in a full circle, as shown. Because the units of measure for s and r are the same, the ratio s/r has no units—it is a real number.

16. Radian Measure Because the measure of an angle of one full revolution is s/r = 2r/r = 2 radians, you can obtain the following. 180 ̊ 90 ̊ 60 ̊

17. Radian Measure These and other common angles are shown below. 30 ̊ 45 ̊ 60 ̊ 180 ̊ 360 ̊ 90 ̊

18. Radian Measure We know that the four quadrants in a coordinate system are numbered I, II, III, and IV. The figure to the right shows whichangles between 0 and 2 lie in each of the four quadrants. Note that angles between 0 and /2are acute angles and angles between /2and  are obtuse angles.

19. Comments on Radian Measure • A radian is an amount of rotation that is independent of the radius chosen for rotation • For example, all of these give a rotation of 1 radian: • radius of 2 rotated along an arc length of 2 • radius of 1 rotated along an arc length of 1 • radius of 5 rotated along an arc length of 5, etc.

20. Coterminal Angles • Two angles are coterminal if they have the same initial and terminal sides. • The angles 0 and 2𝜋 are coterminal. • You can find an angle that is coterminal to a given angle θ by adding or subtracting 2𝜋. • For negative angles subtract 2𝜋 and for positive angles add 2𝜋.

21. Coterminal Angles Two angles are coterminal when they have the same initial and terminal sides. For instance, the angles 0 and 2 are coterminal, as are the angles /6 and 13/6. You can find an angle that is coterminal to a given angle  by adding or subtracting 2 (one revolution), as demonstrated in Example 1. A given angle  has infinitely many coterminal angles. For instance,  = /6 is coterminal with where n is an integer.

22. Example 1 – Finding Coterminal Angles a. For the positive angle 13/6, subtract 2 to obtain acoterminal angle

23. Example 1 – Finding Coterminal Angles cont’d b. For the negative angle –2/3, add 2 to obtain a coterminal angle

24. Find 2 coterminal angles to (one positive and one negative). Example 2 – Finding Coterminal Angles Find a positive coterminal angle to 20º Find a negative coterminal angle to 20º

25. Your Turn: • Find two Coterminal Angles (+ and -) positive negative

26. Complementary & Supplementary Angles Two positive angles  and  are complementary (complements of each other) when their sum is /2. Two positive angles are supplementary (supplements of each other) when their sum is . Complementary angles Supplementary angles

27. Example: Complementary Angles: Two angles whose sum is 90 or (𝜋/2) radians. Supplementary Angles: Two angles whose sum is 180 or 𝜋 radians.

28. Your Turn: Complement Supplement Supplement Complement - None Two positive angles are complementary if their sum is 𝜋/2. Two positive angles are supplementary if their sum is 𝜋. Find the Complement and Supplement

29. Degree Measure A second way to measure angles is in degrees, denoted by the symbol . A measure of one degree (1) is equivalent to a rotation of of a complete revolution about the vertex. To measure angles, it is convenient to mark degrees on the circumference of a circle.

30. Degree Measure So, a full revolution (counterclockwise) corresponds to 360, a half revolution to 180, a quarter revolution to 90, and so on. Because 2 radians corresponds to one complete revolution, degrees and radians are related by the equations 360 = 2 rad and 180 =  rad. From the latter equation, you obtain and which lead to the conversion rules in the next slide.

31. Degree Measure

32. Degree Measure When no units of angle measure are specified, radian measure is implied. For instance,  = 2, implies that  = 2 radians.

33. Example – Converting from Degrees to Radians a. b. Multiply by  rad/180. Multiply by  rad/180.

34. Example – Converting from Radians to Degrees Multiply by 180/ rad. Multiply by 180/ rad.

35. Your Turn: • Convert from degrees to radians • 135° • 540° • -270° • Convert from radians to degrees

36. Degrees Radians Degrees Radians Exact Approximate Exact Approximate 0 0 0 90 1.57 30 .52 180  3.14 45 .79 270 4.71 60 1.05 360 2 6.28 Equivalent Angles in Degrees and Radians

37. Equivalent Angles in Degrees and Radians cont.

38. A Sense of Angle Sizes See if you can guess the size of these angles first in degrees and then in radians. You will be working so much with these angles, you should know them in both degrees and radians.

39. Applications

40. Vocabulary • Arc • Sector • Tangent • Secant • Chord • Segment

41. Arc, Sector, Segment • Arc: any unbroken part of the circumference • Measured in radians • Sector: a plane figure bounded by two radii and the included arc • Segment: a part cut off from a circle by a line, as a part of a circular area contained by an arc and its chord

42. Intercepted Arcs • An intercepted arc is the arc that is formed when segments intersect portions of a circle and create arcs. These segments in effect 'intercept' parts of the circle.

43. Major Arcs/Minor Arcs • The bigger one is major (≥π). The smaller is minor(<π). • Arc HN is the minor arc • Arc HKN is the major arc

44. Arc Lengths and Central Angles of a Circle • Given a circle of radius “r”, any angle with vertex at the center of the circle is called a “central angle”. • The portion of the circle intercepted by the central angle is called an “arc” and has a specific length called “arc length” represented by “s”. • From geometry it is know that in a specific circle the length of an arc is proportional to the measure of its central angle. • For any two central angles, and , with corresponding arc lengths and :

45. Development of Formula for Arc Length • Since this relationship is true for any two central angles and corresponding arc lengths in a circle of radius r: • Let one angle be with corresponding arc length and let the other central angle be , a whole rotation, with arc length .

46. Applications The radian measure formula,  = s/r, can be used to measure arc length along a circle.

47. Example – Finding Arc Length A circle has a radius of 4 inches. Find the length of the arc intercepted by a central angle of 240.

48. Example – Solution To use the formula s = r, first convert 240 to radian measure.

49. Example – Solution cont’d Then, using a radius of r = 4 inches, you can find the arc length to be s = r Note that the units for r determine the units for rbecause  is given in radian measure, which has no units.

50. Your Turn: Finding Arc Length • A circle has radius 18.2 cm. Find the length of the arc intercepted by a central angle having the following measure: