# Angles and Their Measure

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1 Trigonometry Lecture Notes Section 5.1 Angles and Their Measure Definitions: A Ray is part of a line that has only one end point and extends forever in the opposite direction. An Angle is formed by two rays that have a common endpoint. One ray is called the initial side and the other is called the terminal side. The common endpoint of an angle s initial side and terminal side is the vertex of the angle. Standard Position: An angle is in Standard Position if Its vertex is at the origin of a rectangular coordinate system and Its initial side lies along the positive x-axis Positive angles are generated by counter clockwise rotation, while negative angles are generated by clockwise rotation. The angles illustrated above lie in Quadrants I (first and last drawing), and III (second drawing). Note not all angles lie in a quadrant.

2 Measuring Angles Using Degrees If the minute hand of a clock goes from the 1 noon position all the way to the 1 midnight position it has traveled 360 degrees, denoted as 360. Special names for angles of certain measure: An Acute angle measures less than 90 degrees. A Right angle measures exactly 90 degrees. An Obtuse angle measures more than 90, but less than 180 degrees. A Straight angle measures exactly 180 degrees. Example 1 Draw each of the following angles in standard position: a. 45 b. 5 c. 90 d. 70 e. 135 f. 405 We refer to angles with the same initial and terminal sides, like examples a. and f., as coterminal angles. An angle of X is coterminal with angles of X + k 360 where k is an integer. Example Find an angle less than 360 degrees that is coterminal with a. 430 b. 10

3 Two positive angles are Complements if their sum is 90 degrees, and two positive angles are Supplements if their sum is 180 degrees. Example 3 Find the complement and supplement of 6 Radians Radians are another way to measure angles. One Radian is the measure of the central angle of a circle that intercepts an arc equal in length to the radius of the circle. See the picture below: The radian measure of any central angle is the length of the intercepted arc divided by the radius of the circle. length of the intercepted arc s θ = = radians radius r

5 We should memorize these diagrams

6 Example 6.5 Convert 63 3'47" into decimal form. Then convert into DMS form. Section 5. Right Angle Trigonometry Triangles have three angles. Some special triangles have a 90 degree angle (just one of course, since the sum of all angles in a triangle must add to 180 degrees). We want to be able to talk about some special names for the sides of these triangles relative to one of the other (non-90 ) angles. Consider the diagram below:

7 Now we can talk about the six trigonometric functions. These functions are like the functions you know of already, such as f ( X ), but the six trig functions have names instead of the single letter names of f, g, and h. Here are the names and their abbreviations: Name Abbreviation Name Abbreviation Sine Sin Cosecant Csc Cosine Cos Secant Sec Tangent Tan Cotangent Cot Here is how we will define these functions:

8 Notice the functions in the left column (sine, cosine, and tangent) are reciprocals of the right column. [Memory aids: Soh, Cah, Toa, or my favorite S: Oscar Had, C: A Heap, T: Of Apples] Example 7 Assume b = 1 and a = 5, then find the value of each of the six trig functions of angle A. Special Angles Example 8 Evaluate the six trigonometric functions for the 45 angle ( 4 π ) below:

9 Example 9 Evaluate sine, cosine, and tangent functions for the 60 or π and the 30 or π below: 3 6 I will require you to memorize the sine and cosine values for the 16 angles shown below on the Unit Circle: Below is another version that is computer drawn:

10 Fundamental Identities The reciprocal identities: 1 sinθ = cscθ 1 cscθ = sinθ 1 cosθ = se cθ 1 secθ = cosθ 1 tanθ = cotθ 1 cotθ = tanθ The quotient identities: sinθ tanθ = cosθ cosθ cotθ = sinθ

11 Example 10 1 Given sinθ = and cosθ = 3, find the value of each of the remaining trigonometric functions. Recall the Pythagorean Theorem for right triangles, triangle: a + b = c, and consider the following Let s divide the equation by c, then we have a c b c + = 1 Which is the same as a b + = 1 c c Relative to θin our drawing this leads us to perhaps the most important the identity in this course: sin θ + cos θ = 1 ***Please note that ( ) sinθ = sin By using the same approach but dividing by either identities. The Pythagorean Identities θ a and b, we can arrive at the following θ θ sin + cos = 1 1+ tan θ = sec θ 1+ cot θ = csc θ Example 11 Given that identity. 3 sinθ = and that θis an acute angle, find the value of cosθ using the Pythagorean 5 Trig Functions and Complements: As mentioned above, the two non 90 angles must add to 90. This makes them complementary. This means that the cosine of one of these angles will always equal the sine of the other angle and vice versa. You will also see this pattern on the unit circle I asked you to memorize. Let s see why, consider the drawing below:

12 Consider the angles θand B What is the sine ofθ? How does that compare with the cosine of B? (Notice that angle B = 90 - θ) The Cofunction Identities (note if using radian measure use π instead of 90 ) ( θ ) ( θ ) ( θ ) sinθ = cos 90 tanθ = cot 90 secθ = csc 90 ( θ ) ( θ ) ( θ ) cosθ = sin 90 cotθ = tan 90 cscθ = sec 90 Example 1 Find a cofunction with the same value as the expression given: a. sin 38 b. csc 3 π Applications Many problems of right angle trigonometry involve the angle made with an imaginary horizontal line. The angle formed by that line and the line of sight to an object that is above the horizontal is called the angle of elevation. If the object is below the horizontal the angle is called the angle of depression.

13 Example 13 A surveyor sights the top of a Giant Sequoya tree in California and measures the angle of elevation to be 3 degrees. The surveyor is standing 75 feet away from the base of the tree. What is the height of the tree? 3 Example A plane must achieve a minimum angle of elevation once it reaches the end of its runway which is 00 yards from a six story building in order to clear the top of the building safely. What angle of elevation must the plane exceed in order to clear the 60 ft tall building in its path? Section 5.3 Trig Functions of Any Angle

14 Let θbe any angle in standard position, and let P ( x, y) r = x + y is the distance from ( ) defined as: = be a point on the terminal side of θ. If 0,0 to( x, y ), as shown below, the six trig functions of θare y sinθ = r r csc θ =, y 0 y cosθ = x r r sec θ =, x 0 x y tan θ =, x 0 x x cot θ =, y 0 y Example 15 Let P = (,5) be a point on the terminal side of θ. Find each of the six trigonometric functions of θ.

15 Example 16 Find the sine function and the tangent function (if possible) at the following four quadrantal angles: a. θ = 0 b. π θ = c. θ = π d. θ = 3π Signs of the Trigonometric Functions Since the six trigonometric function all depend on the quantities x, y, and r and since r is always positive, the sign of a trigonometric function depends upon the quadrant in whichθlies. For example, { } in quadrant II we have angles of measures that are in the following set θ θ ( 90,180 ). This means that the sines of all of those angles are positive (since all y s are positive in quad II), the cosines of all of those angles are negative (since all x s are negative in quad II), and finally the tangents in quadrant II are always negative (since x and y have different signs). Example 17 If tan θ> 0 and cosθ< 0, name the quadrant in which angle θlies. Example 18 Given tanθ = and cosθ > 0, find cosθ and cscθ. 3 Reference Angles We will often evaluate the trigonometric functions of positive angles greater than 90 and all negative angles by making use of a positive acute angle. This angle is called a reference angle. Definition of a Reference Angle Let θbe a non-acute angle in standard position that lies in a quadrant. Its reference angle is the positive acute angle θ formed by the terminal side of θand the x-axis.

16 If 90 < θ < 180, then β = 180 θ then β = θ 180 If 180 < θ < 70, If 70 < θ < 360, then β = 360 θ Example 19 Find the reference angle, β, for each of the following angles: a. θ = 345 b. 5π θ = c. θ = d. θ =.5 Using Reference Angles to Evaluate Trigonometric Functions The value of a trigonometric function of any angle θis found as follows: 1. Find the appropriate reference angle, β.. Determine the required function value for β (i.e.-sine, cos, tan, ). 3. Use the quadrant that θlies in to determine the appropriate sign for the answer from step. Example 0 Use a reference angle to determine the exact value of each of the following trig functions: a. sin135 b. Section cos 3 π π c. cot 3 Trigonometric Functions of Real Numbers; Periodic Functions

17 The unit circle we have been working with has the equation x + y = 1, if we were to solve this equation of a circle for y this equation does not form a function of x (clearly, it would fail the vertical line test). However, recall our arc length formula s = rθ, for the unit circle, r is equal to one. This means our arc length formula will give us this simpler relationship: s = 1θ (or just s = θ ). The length of the intercepted arc is equal to the radian measure of the angle. Now let s change up the notation a bit. We will call the angle t instead of θin this section. Now consider the diagram below: For each real number value for t, there is a corresponding point P = (x,y) on the unit circle. We can now define the cosine function at t to be the x-coordinate of P and the sine function at t to be the y- coordinate of P. Namely: x = cos t and y = sin t. The Definitions of the Trigonometric Functions in Terms of a Unit Circle If t is a real number and P = (x, y ) is a point on the unit circle that corresponds to t, then: sin t y = y cos t = x tan t =, x 0 x 1 csc t =, y 0 y 1 x sec t =, x 0 cot t =, y 0 x y Example 1 Use the figure to find the values of the trig functions at t = 8 π

18 Domain and Range of the Sine and Cosine Functions Let s consider the unit circle definitions of sine and cosine, namely: cos t = x and sin t = y. Since t corresponds to an arc length along the unit circle it can be any real number. This means that the domain for these two functions must be all real numbers. Now, because the radius of the unit circle is one, we have restrictions on the values that are possible for both y and x, so the ranges will be given by: [ 1,1]. That is 1 cos t 1, and 1 sin t 1. Even and Odd Trigonometric Functions Recall that an even function is a function that has the following property: f ( x) = f ( x) Also, recall that an odd function is a function that has the following property: f ( x) = f ( x) Now, consider two points on the unit circle P : ( cos t,sin t) and Q :( cos( t),sin( t) ) drawing of two such points:, I have created a Notice how the x value is the same for both points in spite of the fact that the point Q uses the angle t, but you can also see that the y values have the opposite sign of each other. cos t = cos( t) This diagram helps us to see that which means that cosine is an even function, and sine sin( t) = sin t is an odd function.

19 Even and Odd Trigonometric Functions The cosine and secant functions are even. cos( t) = cost sec( t) = sect The sine, cosecant, tangent, and cotangent functions are odd. sin( t) = sin t csc( t) = csct tan( t) = tan t cot( t) = cot t Example π Find the exact value of a. cos (- 45 ) and b. tan( ). 3 Periodic Functions A function f is periodic if there exists a positive number p such that f ( t + p) = f ( t) for all t in the domain of f. The smallest number p for which f is periodic is called the period of f. Periodic Properties of the Sine and Cosine Functions ( t + π ) = t ( t π ) sin sin cos + = cost The sine and cosine functions are periodic functions and have period π. Example 3 Find the exact value of: a. tan 40 and b. 9 sin 4 π Periodic Properties of the Tangent and Cotangent Functions tan ( t + π ) = tan t cot ( t π ) + = cot t The tangent and cotangent functions are periodic functions and have period π. Repetitive Behavior of the Sine, Cosine, and Tangent Functions

20 For any integer n and real number t, ( t + π n) = t, cos ( t + π n) = cost, and tan ( π ) sin sin Section 5.5 Graphs of Sine and Cosine Functions t + n = tan t The Graph of y = sin x We can graph the trig functions in the rectangular coordinate system by plotting points whose coordinates satisfy the function. Because the period of the sine function is π, we will graph the function on the interval [0, π ]. Table of values (x, y) on y = sin x X 0 π 6 π 3 π π 5π π 7π 4π 3π 5π 11π π y Below is a graph of the sine curve which uses two periods instead of just the one we have above. From the graph of the sine wave we can see the following things: The domain is the set of all real numbers.

21 The range consists of all numbers from [-1, 1] The period is π. This function is an odd function which can be seen from a graph by observing symmetry with respect to the origin. Graphing Variations of y=sinx To graph variations of y = sinx by hand, it is helpful to find x-intercepts, maximum points, and minimum points. There are three intercepts, so we can find these five points by using the following scheme. Let x 1 = the x value where the cycle begins. Then for all i in [,5] the ith period point can be found using: xi = xi We can stretch or shrink the graph by adding a coefficient other than one to the sine graph: y = Asin x This is the graph of y = sinx, compare it to the graph below of y = sinx:

22 In general the graph of y = Asinx ranges between A and A. We call A the amplitude of the graph. If it is A > 1 the graph is stretched, if A < 1 it is shrunk. Graphing Variations of y=sinx Identify the amplitude and the period. Find the values of x for the five key points the three x-intercepts, the maximum point, and the minimum point. Start with the value of x where the cycle begins and add quarter-periods that is, period/4 to find successive values of x. Find the values of y for the five key points by evaluating the function at each value of x from step. Connect the five key points with a smooth curve and graph one complete cycle of the given function. Extend the graph in step 4 to the left or right as desired. Example 4 Determine the amplitude of y = 1/ sin x. Then graph y = sin x and y = 1/ sin x for 0 < x < π. Step 1 Identify the amplitude and the period. The equation y = 1/ sin x is of the form y = A sin x with A = 1/. Thus, the amplitude A = 1/. This means that the maximum value of y is 1/ and the minimum value of y is -1/. The period for both y = 1/ sin x and y = sin x is π.

23 Step Find the values of x for the five key points. We need to find the three x-intercepts, the maximum point, and the minimum point on the interval [0, π]. To do so, we begin by dividing the period, π, by 4. Period/4 = π/4 = π/ We start with the value of x where the cycle begins: x = 0. Now we add quarter periods, π /, to generate x-values for each of the key points. The five x-values are x = 0, x = π/, x = π, x = 3 π/, x = π Step 3 Find the values of y for the five key points. We evaluate the function at each value of x from step. (0,0), (π/, 1/), (π,0), (3 π/, -1/), (π, 0) Step 4 Connect the five key points with a smooth curve and graph one complete cycle of the given function. The five key points for y = 1/sin x are shown below. By connecting the points with a smooth curve, the figure shows one complete cycle of y = 1/sin x. Also shown is graph of y = sin x. The graph of y = 1/sin x shrinks the graph of y = sin x. Amplitudes and Periods The graph of y = A sin Bx has amplitude = A and period = π. B

24 Example 5 Determine the amplitude and period of y = 3sinx, then graph the function for 0 x π. Steps 1. Determine the amplitude and the period using the form y = Asin Bx. period. Divide the period by four and find the five key points using xi = xi Get your resulting y-values after plugging in the above x-values. 4. Extend the graph as you wish The Graph of y = Asin(Bx - C) The graph of y = A sin (Bx C) is obtained by horizontally shifting the graph of y = A sin Bx so that the starting point of the cycle is shifted from x = 0 to x = C B. The number C B phase shift. is called the

25 Example 6 Determine the amplitude, period, and phase shift of y = sin(3x-π) Solution: y = A sin (Bx C) Amplitude = A = period = π/b = π/3 phase shift = C/B = π/ y = sin(3x- π) The Graph of y = AcosBx We graph y = cos x by listing some points on the graph. Because the period of the cosine function is π, we will concentrate on the graph of the basic cosine curve on the interval [0, π]. The rest of the graph is made up of repetitions of this portion. Below is a graph of the cosine curve. From the graph we can see the following things:

26 The domain is the set of all real numbers. The range consists of all numbers from [-1, 1] The period is π. This function is an even function which can be seen from a graph by observing symmetry with respect to the y-axis. Note the similarity between the graphs of Sine and Cosine. In fact, they are related by the π following identity: cos x = sin x + Like the sine graph, the graph of y = A cos Bx has amplitude = A & period = π/b. Example 7 π Determine the amplitude and period of y = 3cos x. Then graph the function for [-4, 4].

27 Steps 1. Determine the amplitude and the period using the form y = Acos Bx. period. Divide the period by four and find the five key points using xi = xi Get your resulting y-values after plugging in the above x-values. 4. Extend the graph as you wish. The Graph of y = Acos(Bx - C) The graph of y = A cos (Bx C) is obtained by horizontally shifting the graph of y = A cos Bx so that the starting point of the cycle is shifted from x = 0 to x = C B. The number C B phase shift. Amplitude = A & the period = π /B. is called the Remember! In the example above, C is not negative. In the general form, there is subtraction in the parentheses. Therefore, C is positive, which makes the function's phase shift to the right. Example 8 Determine the amplitude, period, and phase shift of y ( x π ) the function. Vertical shifts of the Sinusoidal Graphs = 1 cos 4 +. Then graph one period of Consider the following forms: y = Asin ( Bx C) + D and cos ( ) y = A Bx C + D The constant D causes vertical shifts in the graphs of the functions. This will change the maximums and minimums so that the maximum becomes D + A and the minimum becomes D A. Example 9 Graph one period of 1 y = cos x 1

28 Section 5.6 Graphs of Other Trigonometric Functions The Graph of y = tanx The graph of tangent is very different from the graphs of sine and cosine. Here are some properties of the tangent function we should consider before graphing: The period isπ. It is only necessary to graph tangent over an interval of lengthπ. After this the graph just repeats. The tangent function is an odd function: Tan(-x) = -Tanx. The graph is therefore symmetric with respect to the origin. The tangent function is undefined at π. Therefore, the graph will have a vertical asymptote at x = π. We obtain the graph of y = tanx using some points on the graph and origin symmetry. The table π below lists some tangent values over the interval 0,. X 0 Y = tan x π 6 π 4 π 5π ( ) 17π π 1.57 ( 89 ) 180 ( 85 ) π Undefine d **Notice how as we approach 90 the graphs increases slowly at first then dramatically. This due to the vertical asymptote at π. Below are some properties of the tangent function and its graph: Period: π Domain: All real numbers except π / + k π, where k an integer Range: All real numbers Symmetric with respect to the origin Vertical asymptotes at all odd multiples of π / Below is the graph of y = tan x:

29 Graphing y = A tan(bx C)

30 Example 30 Graph x y = tan for π < x < 3π Find two consecutive asymptotes by solving π Bx C = and Bx C = π. Example 31 Identify an x-intercept midway between the asymptotes (average the asymptotes x s). Find points ¼ and ¾ of the way between consecutive asymptotes. These points have y- coordinates A and A. Connect these points with a smooth curve. Graph two full periods of π y = tan x + 4 Find two consecutive asymptotes by solving π Bx C = and Bx C = π. Identify an x-intercept midway between the asymptotes (average the asymptotes x s). Find points ¼ and ¾ of the way between consecutive asymptotes. These points have y- coordinates A and A. Connect these points with a smooth curve. The Cotangent Curve: The Graph of y = cotx and Its Characteristics

31 Graphing Variations of y = cot x Graphing y=acot(bx-c) Example 3 Graph y = 3cot x Find two consecutive asymptotes by solving Bx C = 0 and Bx C = π. Identify an x-intercept midway between the asymptotes (average the asymptotes x s). Find points ¼ and ¾ of the way between consecutive asymptotes. These points have y- coordinates A and -A. Connect these points with a smooth curve.

32 The Cosecant Curve: The Graph of y = cscx and Its Characteristics For the Cosecant and Secant curves it is helpful to graph the related Sine and Cosine curves first. X-intercepts on the sine curve correspond to vertical asymptotes on the cosecant curve. A maximum on the sine curve represents a minimum on the cosecant curve. A minimum on the sine curve represents a maximum on the cosecant curve.

33 Example 33 The Secant Curve: The Graph of y=secx and Its Characteristics X-intercepts on the cosine curve correspond to vertical asymptotes on the secant curve. A maximum on the cosine curve represents a minimum on the secant curve. A minimum on the cosine curve represents a maximum on the secant curve.

34 Example 34 Graph x x y = 3sec for π < x < 5π (Hint: use the graph of y = 3cos ) Section 5.7 Inverse Trigonometric Functions Recall that in order for a function to have an inverse it must pass the horizontal line test (i.e.-it must be one-to-one). Also, the inverse function s graph will be the reflection of the original function about the line y = x. Since the trig functions would fail to be one-to-one on their entire domain due to their periodic nature among other things, we must restrict their domains to be able to find their inverses. The Inverse Sine Function The inverse sine function, denoted by sin -1, is the inverse of the restricted sine function: y = sin x, - π / < x < π /. Thus, y = sin -1 x means sin y = x, at what angle is the sine equal to x? where - π / < y < π / and 1 < x < 1. We read y = sin -1 x as y equals the inverse sine at x. We can graph the inverse of the sine function with the above restricted domain by simple reversing the points. For example, (x, y) is switched to (y, x). Finding Exact Values of sin -1 x

35 Let θ = sin -1 x. Rewrite step 1 as sin θ = x. Use the exact values from the unit circle to find the value of θ in [- π /, π /] that satisfies sin θ = x. Example 35 Find the exact value of sin -1 (1/) The Inverse Cosine Function The inverse cosine function, denoted by cos -1, is the inverse of the restricted cosine function y = cos x, 0 < x < π. Thus, y = cos -1 x means cos y = x, where 0 < y < π and 1 < x < 1. Example 36 Find the exact value of cos -1 (- 3 /) The Inverse Tangent Function The inverse tangent function, denoted by tan -1, is the inverse of the restricted tangent function y = tan x, -π/ < x < π/. Thus, y = tan -1 x means tan y = x, where - π / < y < π / and < x <. Example 37 Find the exact value of 1 tan 3

36 Inverse Properties The Sine Function and Its Inverse sin (sin -1 x) = x for every x in the interval [-1, 1]. sin -1 (sin x) = x for every x in the interval [-π/,π/]. The Cosine Function and Its Inverse cos (cos -1 x) = x for every x in the interval [-1, 1]. cos -1 (cos x) = x for every x in the interval [0, π]. The Tangent Function and Its Inverse tan (tan -1 x) = x for every real number x tan -1 (tan x) = x for every x in the interval (-π/,π/). Example 38 Find the exact values if possible of: 1 a. cos( cos 0.6) b. π sin sin c. cos ( cos π ) Example 39 Find the exact value of cos tan Example 40 Find the exact value of 1 1 cot sin 3. Example 41 Calculators and the Inverse Trigonometric Functions Use your calculator to find 1 1 sin 4 1 and tan ( 9.65) in radian mode.

37 Graphs of the Three Basic Trigonometric Functions

38 Arc tangent

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