MTH 112 Elementary Functions Chapter 5 The Trigonometric Functions Section 4 – Radians, Arc Length, and Angular Speed
Angle Measurement – Degrees Full rotation from the positive x-axis to the positive x-axis is 360°. Why? Therefore, rotation from the positive x-axis to … the positive y-axis is 90°. the negative x-axis is 180°. the negative y-axis is 270°.
Problem: Find the length of an arc of a circle of radius r that is intercepted by a central angle of degree measure . a r r Circumference of the circle is 2 r. The arc is /360 th of the circle. Therefore, a is /360 th of 2 r.
Angle Measurement – Radians The measure of the angle in radians is the length of the arc intercepted by the angle, divided by the radius. = a / r radians x 2 + y 2 = r 2 a r r Since any circle produces the same results, a unit circle is used (i.e. r = 1).
Angle Measurement: Radians vs. Degrees With a right angle … 90° = / 2 radians 1 = / 180 radians a° = ( / 180)a radians b radians = (180 / )b° Circumference of a Unit Circle: 2 22 90° 1 x 2 + y 2 = 1
Angle Measurement: Radians vs. Degrees Degree and DMS measurements are designated using the symbols °, ‘, and “. Examples: 25.47° 25°28’12” Radian measurements do not use any symbols. Examples: 283 / 2000 NOTE: All of the examples above are the same measurement (third one is approximate).
DegreesRadians 0°0°0 30° / 6 45° / 4 60° / 3 90° / 2 Common Angles: Degrees vs. Radians First Quadrant
DegreesRadians 90° / 2 120° 2 / 3 135° 3 / 4 150° 5 / 6 180° Common Angles: Degrees vs. Radians Second Quadrant
DegreesRadians 180° 210° 7 / 6 225° / 4 240° 4 / 3 270° 3 / 2 Common Angles: Degrees vs. Radians Third Quadrant
DegreesRadians 270° 3 / 2 300° 5 / 3 315° 7 / 4 330° 11 / 6 360° 22 Common Angles: Degrees vs. Radians Fourth Quadrant
The Unit Circle - Summarized
The Unit Circle – Summarized Axis Points
The Unit Circle – Summarized k /6 Points
The Unit Circle – Summarized k /4 Points
The Unit Circle – Summarized k /3 Points
Other Angles Negative Angles Go clockwise (e.g. - /6 is the same point as 11 /6) Ordered pairs are the same Angles Greater than 2 Repeats the same points General Rule: “Add or subtract multiples of 2 to get the radian measure between 0 and 2 .”
Problem: Find the length of an arc of a circle of radius r that is intercepted by a central angle of radian measure . a r r Circumference of the circle is 2 r. The arc is /(2 ) th of the circle. Therefore, a is /(2 ) th of 2 r.