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Radians We have previously measured angles in degrees, but now we introduce a new way to measure angles--in radians. The arc length spanned, or cut off,

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Presentation on theme: "Radians We have previously measured angles in degrees, but now we introduce a new way to measure angles--in radians. The arc length spanned, or cut off,"— Presentation transcript:

1 Radians We have previously measured angles in degrees, but now we introduce a new way to measure angles--in radians. The arc length spanned, or cut off, by an angle is shown next: An angle of 1 radian is defined to be the angle, in the counterclockwise direction, at the center of a unit circle which spans an arc of length 1. An angle of 1 radian is approximately equal to If an angle is referred to without units, radians is assumed. x y

2 The radian measure of a positive angle is the length of arc spanned by the angle in a unit circle. For a negative angle, the radian measure is the negative of the arc length. To convert from degrees to radians, multiply the degree measure by To convert from radians to degrees, multiply the radian measure by Examples. Problem. Convert 4 radians to degrees. In which quadrant is this angle?

3 Arc Length The arc length, s, spanned in a circle of radius r by an angle of radians is given by Problem. What length of arc is cut off by an angle of 120 degrees on a circle of radius 12 cm? Solution. First convert to radians, then multiply by 12 to get an arc length of x y

4 Effect of tire wear on mileage The odometer in your car measures the mileage travelled. The odometer uses the angle that the axle turns to compute the mileage s in the formula Question. With worn tires, are the actual miles travelled more or less than the odometer miles?

5 Values of Sine and Cosine The values for sine and cosine in the following table of “special” angles (see pg 314 of text) should be memorized: You should be able to use reference angles (see next slide) to find the values of sine and cosine for angles which are not in the first quadrant. For example,

6 Reference Angles For an angle corresponding to the point P on the unit circle, the reference angle of is the angle between the line joining P to the origin and the nearest part of the x-axis. A reference angle is always between that is, between (See page 346 of the textbook.) Example. Find the reference angle of x y

7 The Unit Circle x y For any angle  : (x,y) = (cos , sin  )

8 Graphs of sin x and cos x, where x is in radians The function sin x is periodic with period The function cos x is periodic with period

9 Red arc length is π/3. Generation of Sine Curve Using Unit Circle Q1Q2

10 Graph of sin x, where x is in radians One cycle of the function sin x.

11 Graph of cos x, where x is in radians One cycle of the function cos x.

12 Graph of ferris wheel function The graph of the function giving your height, y = f(x), in feet, as a function of the angle x, measured in radians from the 3 o’clock position, is shown next. Period = 2π Midline y = 225 ft. f(x)= sin x

13 Sinusoidal Functions We consider functions which can be expressed in the form: where A, B, h, and k are constants and t is measured in radians. The graphs of these functions resemble those of sine and cosine. They may start with sine or cosine and then shift, flip, or stretch the graph. Which of these transformations are to be applied is determined by the values of the constants A, B, h, and k as follows: |A| is the amplitude h is the horizontal shift y = k is the midline |B| is the number of cycles completed in

14 One Cycle of Assume A, B, h, and k are positive.

15 The London Eye Ferris wheel function as a sinusoidal function Problem. Use the sinusoidal function to represent your height f(t) above ground while riding the Ferris wheel. Solution. The diameter of the Ferris wheel is 450 feet so the midline is k = 225 and the amplitude, A, is also 225. The period of the Ferris wheel is 30 minutes, so The graph is shifted to the right by t = 7.5 minutes since we reach y = 225 (the 3 o’clock position) when t = 7.5. Thus, the horizontal shift is h = 7.5, and the formula for f(t) is: where t is in minutes and height is in feet.

16 Height of capsule on London Eye Ferris wheel

17 Phase shift Assume that a sinusoidal function has been given. The phase shift is defined as Then since it follows that the ratio of the phase shift to equals the fraction of a period by which the sinusoidal function is shifted. For sinusoidal functions written in the form

18 Phase shift, continued Phase shift is significant because we often want to know if two waves reinforce or cancel each other. For two waves of the same period, a phase shift of 0 or 2  tells us that the two waves reinforce each other while a phase shift of  tells us that the two waves cancel. Example. The graph of is the same as the graph of but shifted  /12 units to the left, which is (  /4)/(2  ) = 1/8 of the period of cos(3t). cos(3t) cos(3t +  /4)

19 Sine with a phase shift of π/2 plot({sin(t),sin(t+Pi/2)},t=-Pi..Pi,color=black); The shifted sine curve is the cosine. Likewise, we can shift the cosine curve right by π/2 to get the sine.

20 Interpreting the shifted sine curve From the previous slide, we have Upon replacing t by –t, we have using the fact that cosine is an even function. In terms of angles of a right triangle, the latter result is: Ө φ a b c

21 The tangent function revisited Suppose P = (x, y) is the point on the unit circle specified by the angle. We define the tangent of, or tan, by tan = y/x. The graphical interpretation of tan is as a slope. In the figure below, the slope m of the line passing from the origin through P is given by

22 Values and graph of the tangent function The values of the tangent function for “special” angles are: The function tan x is periodic with period, and its graph is next. It has a vertical asymptote when x is an odd multiple of

23 Reciprocals of the trig functions The reciprocals of the trig functions are given special names. Where the denominators are not equal to zero, we define: The graphs of these new functions can be easily obtained from the graphs of the functions of which they are the reciprocals.

24 Summary of Trigonometric Relationships Sine and Cosine functions Pythagorean Identity Tangent and Cotangent Secant and Cosecant

25 Inverse Trigonometric Functions Solving equations involving trigonometric functions can often be reduced (after some algebraic manipulation) to an equation similar to the following example: Find a value of t radians satisfying cos t = 0.4. In order to find a value of t as in the above example, we can use a calculator which has support for inverse cosine. Such a calculator in radian mode allows us to find the required value of t by pressing the button labeled as shown below: Pressing this button yields cos -1 (0.4) 1.16, where cos(1.16) 0.4. cos -1

26 The Inverse Cosine Function. The inverse cosine function, also called the arccosine function, is denoted by cos -1 y or arccos y. We define In other words, if t = arccos y, then t is the angle between 0 and whose cosine is y. The inverse cosine has domain In conjunction with the inverse cosine, we may have to use reference angles to find answers which are not in Example. cos -1 (0.4) 1.16 gives us an answer in the first quadrant. Suppose we want an answer in the fourth quadrant. We simply subtract the reference angle from to obtain t 5.12.

27 The Inverse Sine Function. The inverse sine function, also called the arcsine function, is denoted by sin -1 y or arcsin y. We define The inverse sine has domain In conjunction with the inverse sine, we may have to use reference angles to find answers which are not in its range. Example. sin -1 (0.707) = gives us an answer in the first quadrant. Suppose we want an answer in the second quadrant. We simply subtract the reference angle from to obtain t =.

28 The Inverse Tangent Function. The inverse tangent function, also called the arctangent function, is denoted by tan -1 y or arctan y. We define The inverse tangent has domain In conjunction with the inverse tangent, we may have to use reference angles to find answers which are not in its range. Example. tan -1 (1.732) = gives us an answer in the first quadrant. Suppose we want an answer in the third quadrant. We simply add the reference angle to to obtain t =

29 While riding the London Eye Ferris wheel, at which times during the first turn is your height = 400 feet? As shown in the text, we must find two solutions for t between 0 and 30 in the equation The first solution is given by the arcsin. We solve for t in the equation This yields t = minutes. The second solution corresponds to another angle on the circle with the same reference angle, 0.891, and a positive value of the sine. This is in the second quadrant, so we have

30 Summary for the Trigonometric Functions Radian measure of an angle was defined and The arc length, s, spanned by an angle of radians in a circle of radius r is Values of sine and cosine for “special” angles in radians were given. Reference angles can be used to find values of sine and cosine for angles in radians which are not in the first quadrant. The sine and cosine functions are periodic with period and their graphs are similar but shifted horizontally. For sinusoidal functions, we related amplitude, period, hor. shift, and midline to the parameters of the function. Phase shift can be used to determine the fraction of a full period by which a sinusoidal graph is shifted. The values of tangent for special angles in radians were given.

31 Summary for the Trigonometric Functions, cont’d. The function tan x is periodic with period and it has vertical asymptotes when x is an odd multiple of Three other trig functions were defined as reciprocals: secant, cosecant, and cotangent. A number of trigonometric relationships were given. The definitions of inverse functions for cosine, sine, and tangent were extended and their domains and ranges were given. Use of the inverse trig functions in solving problems was illustrated.


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