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Sullivan Algebra and Trigonometry: Section 7.1 Angles and Their Measures Objectives of this Section • Convert Between Degrees, Minutes, Seconds, and Decimal Forms for Angles • Convert From Degrees to Radians, Radians to Degrees • Find the Arc Length of a Circle •Find the Area of a Sector of a Circle A ray, or half-line, is that portion of a line that starts at a point V on the line and extends indefinitely in one direction. The starting point V of a ray is called its vertex. V Ray If two lines are drawn with a common vertex, they form an angle. One of the rays of an angle is called the initial side and the other the terminal side. Vertex Initial Side Counterclockwise rotation Positive Angle Vertex Initial Side Clockwise rotation Negative Angle Vertex Initial Side Counterclockwise rotation Positive Angle An angle is said to be in standard position if its vertex is at the origin of a rectangular coordinate system and its initial side coincides with with positive x - axis. y Terminal side Vertex Initial side x When an angle is in standard position, the terminal side either will lie in a quadrant, in which case we say lies in that quadrant, or it will lie on the x-axis or the y-axis, in which case we say is a quadrantal angle. y y x x is a quadrantal angle lies in Quadrant III Angles are commonly measured in either Degrees or Radians The angle formed by rotating the initial side exactly once in the counterclockwise direction until it coincides with itself (1 revolution) is said to measure 360 degrees, abbreviated 360 . Terminal side Initial side Vertex 1 One degree, 1 , is revolution. 360 1 A right angle is an angle of 90 , or 4 revolution. Terminal side Vertex Initial side 1 90 angle; revolution 4 A straight angle is an angle of 180 , 1 or revolution. 2 Terminal side Vertex Initial side 1 180 angle; revolution 2 Draw -135 angle. Draw aa-135 angle. y Vertex Initial side x 135 One minute, denoted, 1 , is defined as 1 degree. 60 One second, denoted 1”, is defined as 1 1 minute, or degree. 60 3600 1 counterclockwise revolution = 360 60 = 1 60 = 1 Convert 30 1255 to a decimal in degrees. 30 12 1 55 1 30 12 55 60 3600 30 0.2 0.015278 30.215278 Convert 45.413 to D M S form. 60 0.413 0.413 24.78 1 60 0.78 0.78 46.8 47 1 45.413 45 24 47 Consider a circle of radius r. Construct an angle whose vertex is at the center of this circle, called the central angle, and whose rays subtend an arc on the circle whose length is r. The measure of such an angle is 1 radian. r r 1 radian For a circle of radius r, a central angle of radians subtends an arc whose length s is s r Find the length of the arc of a circle of radius 4 meters subtended by a central angle of 2 radians. r 4 meters and = 2 radians s r 42 8 meters The area A of the sector of a circle of radius r formed by a central angle measured in radians is 1 2 A r 2 Example: Find the area of the sector of the circle of radius 4 inches formed by an angle of 45 degrees. 45 45 rad 180 4 1 2 A (4 ) 2 4 A 2 6.28 in 2 1 revolution = 2 radians 180 radians 1 degree = radian 180 180 1 radian = degrees Convert 135 to radians. radian 135 135 180 3 radian 4 2 Convert - radians to degrees. 3 2 2 180 radians = 3 3 120

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posted: | 12/2/2011 |

language: | English |

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