Trigonometry mathematics, Study notes of Mathematics

Download Trigonometry mathematics and more Study notes Mathematics in PDF only on Docsity! An Introduction to Trigonometry P.Maidorn I. Basic Concepts The trigonometric functions are based on the unit circle, that is a circle with radius r=1. Since the circumference of a circle with radius r is C=2Br, the unit circle has circumference 2B. For any point (x,y) on the unit circle, the associated angle 2 can be measured in two different ways: 1. degree measure: in this case the circumference is divided into 360 equal parts, each part has measure one degree (written 1 ). A right angle, for example, is a 90o o angle. Positive angles are measured in the counter-clockwise direction. 2. radian measure: radian measure is defined as the actual length of the arc between the points (1,0) and (x,y). One entire revolution (i.e. 360 ) hence has a radian measureo of 2B. A right angle (that is a quarter of one revolution) would have radian measure B/2. Note that the angle is simply denoted “B/2”, not “B/2 radians”. One can easily convert between these two measures by keeping in mind that a 180 angleo (in degrees) is equivalent to a B angle (in radians). Note that angles in Calculus-related problems are usually denoted in radian measure, hence it is important to be comfortable with this measurement. Examples: 1. A 270 angle is 3/2 times a 180 angle, hence in radian measure the angle would beo o denoted 3B/2. 2. A 7B/5 angle would simply have degree measure 7/5 times 180 , i.e. 252 .o o Exercises: Convert each angle to radians. a) 120 b) 315 c) -420o o o Convert each angle to degrees, to the nearest tenth of a degree. d) -2B/3 e) 3B f) 4.52 Turning the above discussion around, each time we choose an angle 2, we find a unique point (x,y) on the unit circle. Hence both “x” and “y” can be considered functions of 2. Since these particular functions are of great importance to both pure and applied mathematics, they are given special names and symbols, and are called the trignometric functions. Specifically: The length “y” is called the sine of the angle 2, and is denoted by y=sin(2). The length “x” is called the cosine of the angle 2, and is denoted by x=cos(2). Other trigonometric functions can be calculated from the sine and cosine functions: the tangent of 2 is defined as tan(2)=sin(2)/cos(2) (or y/x), the secant of 2 is defined as sec(2)=1/cos(2), the cosecant of 2 is defined as csc(2)=1/sin(2), and the cotangent of 2 is defined as cot(2)=cos(2)/sin(2). III. Other Angles Once you know how to find the trigonometric functions for the above special angles, it is important to learn how to extend your knowledge to any angle that is based on one of 0, B/6, B/4, B/3 or B/2, such as for example 2B/3, -B/6, 7B/4, -5B/2, and others. Let’s examine the angle 2=-B/3. Clearly, it is somehow related to the angle B/3. Draw a unit circle for both angles side-by-side, and indicate the sine and cosine on them: Clearly, the right-angle triangles that are formed are identical, except that they are mirror images of each other. The cosine (“x”) in both cases is the same, hence we know that cos(-B/3) is identical to cos(B/3), that is cos(-B/3)=½. The sine (“y”) is the same length, but has opposite sign (it is negative). Since sin(B/3)=/3/2, then sin(-B/3)=-/3/2. In each of the examples below, proceed with the same method. It is imperative that you draw the unit circle each time until you become comfortable with these types of questions. Examples: 1. Find sin(5B/6). The triangle formed by 2=5B/6 is identical to that formed by 2=B/6, except that it is reflected across the y-axis. The sine of 5B/6 is hence the same length, and has the same sign, as the sine of B/6. Therefore sin(5B/6)=½. 2. Find sec(-11B/4). To find the secant of an angle, remember to find the cosine first and take its reciprocal. The cosine of -11B/4 is related to the cosine of B/4. The angle -11B/4 is reached by completing one entire clockwise revolution (which equals -2B, or -8B/4) and then adding another -3B/4. Compare the angle -3B/4 to B/4. The cosine is the same length in each case, but has opposite sign. Since cos(B/4) = /2/2, cos(-11B/4) = -/2/2, and hence sec(-11B/4) = -2//2 = - /2. Exercises: Calculate a) sin(-B/4) b) sin(5B/2) c) cos(11B/6) d) cos(13B/4) e) cos(19B/6) f) sin(-510 )o IV. Using the Calculator If you need to calculate the sine, cosine, or tangent of an angle other than the ones discussed above, you may need to use your calculator. Consult your calculator manual, if necessary, on how to use the trigonometric functions. You should be aware of two facts: 1. In most cases, your calculator will not give you exact answers, but rather decimal approximations. For example, your calculator will tell you that the sine of a 45o angle is approximately .70710678, rather than giving you the exact answer /2/2. 2. You need to set your calculator to the appropriate angle measure, degrees or radians. Otherwise, your calculator might return the sine of the angle 2.3.1415... degrees, when you enter “sin(B)” looking for the sine of 2=180 .o Exercises: Use your calculator to compute, rounding to four decimal places: a) sin(53.7 ) b) cos(11B/7) c) csc(-32 )o o V. Right-Triangle Applications In order to apply the trigonometric functions based on the unit circle to right triangles of any size, it is important to understand the concept of similar triangles. Two triangles are said to be similar if the ratio of any two sides of one triangle is the same as the ratio of the equivalent two sides in the other triangle. As a result, similar triangles have the same “shape”, but might differ in size. For example, the sides in the triangles below have the same ratios to each other. Consider the right triangle inscribed in the unit circle associated with an angle 2. We can calculate the length of the side adjacent to the angle 2 (i.e. cos(2)) and the length of the side opposite the angle 2 (i.e. sin(2)). Since the unit circle has radius one, the hypotenuse of these triangles is always equal to one. If we were given a triangle with identical angle 2 but with a hypotenuse twice the length, each of the other sides would be twice the length as well, as the triangles are similar. We can use this fact to now compute side-lengths of any right triangle, if the angle 2 and one side-length are known. In general, we have sin(2) = opposite / hypotenuse and cos(2) = adjacent / hypotenuse and since tan(2)=sin(2)/cos(2), we have tan(2) = opposite / adjacent. Are there any other such angles? By remembering that we can always add or subtract one complete revolution (2B) from an angle to end up in the same position, we can in fact generate 3infinitely many such angles. For example, 2 = B/6 + 2B = 13B/6 is another such angle, as is 42 = 5B/6 - 4B = -19B/6. Hence all angles 2 = B/6 + 2kB and 2 = 5B/6 + 2kB for any k0I (that is k can be any integer ...-3,-2,-1,0,1,2,3,... ) are solutions to the equation sin(x)=½. Exercises: Find all “x” such that a) cos(x) = /3/2 b) cos(x) = -/3/2 c) sin(x) = -1 d) sin(x) = - ½ e) cos(x) = - ½ f) tan(x) = 1 VII. Trigonometric Identities There are several trigonometric identities, that is equations which are valid for any angle 2, which are used in the study of trigonometry. From the unit circle, we have already seen that the cosine of an angle is identical to the cosine of the associated negative angle, that is cos(-2)=cos(2) for any angle 2. (1) Similarly, sin(-2)=-sin(2) for any angle 2. (2) For example, sin(B/6) = ½ and sin(-B/6) = -½. Also from the unit circle, which has equation x +y =1, we can substitute x=cos(2) and2 2 y=sin(2) to obtain the identity sin (2)+cos (2)=1, for any angle 2. (3)2 2 Another useful trigonometric identity concerns the sum of two angles A and B. We have: sin(A+B) = sin(A) cos(B) + sin(B) cos(A) (4) and cos(A+B) = cos(A) cos(B) - sin(A) sin(B) (5) for any angles A and B. Note that you cannot simply “distribute” the sine through a sum. It is false to state that, for example, sin(A+B)=sin(A) + sin(B). The above five identities can be used to derive many other useful identities, which then no longer need to be memorized. Examples: 1. Dividing equation (3) by cos (2), we obtain the identity2 1 + tan (2) = sec (2).2 2 2. Identities involving the difference of two angles A-B can be obtained by combining equation (4) or (5) with equations (1) and (2). As an example, sin(A-B) = sin(A+(-B)) = sin(A) cos(-B) + sin(-B) cos(A) using (4) = sin(A) cos(B) - sin(B) cos(A) using (1) and (2) on the left and right term respectively. 3. The double-angle formula cos(22) = 2 cos (2) -1 can be derived using equation (5) 2 and setting both A=2 and B=2: cos(22) = cos (2 + 2) = cos(2)cos(2) - sin(2)sin(2) using (5) = cos (2) - sin (2)2 2 Now, re-arrange (3) to obtain sin (2)=1-cos (2) and substitute:2 2 cos(22) = cos (2) - (1-cos (2))2 2 = 2cos (2) - 1.2 4. Find all “x” such that 2sin (x) + cos(x) = 1.2 To solve this trigonometric equation, we will first need to simplify it using the trigonometric identities. Using (3) we can substitute sin (x)=1-cos (x) and obtain an equation that2 2 only involves cosines: 2 - 2cos (x) + cos(x) = 12 or 2cos (x) - cos(x) -1 = 0.2 The above is a quadratic equation in cos(x), and can be factored (2cos(x)+1) (cos(x)- 1) = 0. As a result, we now need to find all “x” such that either 2cos(x) + 1 = 0, i.e. cos(x)=-½, or cos(x)-1 =0, i.e. cos(x)=1. From the unit circle, cos(x)=1 when x= 0 +2kB, k0I, i.e. x=2kB, k0I Further, cos(x)=-½ when x=2B/3+2kB or x=4B/3+2kB , k0I. The solution to the given equation is hence the set of all x-values listed above. Exercises: Derive the following equations using equations (1) through (5) only: a) cos(A-B) = cos(A)cos(B) + sin(A)sin(B) b) sin(2A) = 2sin(A)sin(B) c) sin(A)sin(B) = -½ [ cos(A+B) - cos(A-B)] Solve the following equations for “x”: d) 1 - cos(2x) = 2sin(x) e) sin (5x -2x +1) = 1 - cos (5x -2x +1)2 3 2 2 3 2 VIII. Graphing Trigonometric Functions Recall that a function y=f(x) is a rule of correspondence between the independent variable (“x”) and the dependent variable (“y”), such that each x-value is associated with one and only one y-value. Since each angle “x” produces only one value for sin(x), cos(x), etc., the relationships y = sin(x), y = cos(x), etc. are functions of “x”. To obtain the graph of the function y = sin(x), trace how the height of the triangle inscribed in the unit circle changes as the angle “x” gradually moves from x=0 to x=2B. For example, the sine graph will start at (0,0) since the sine of zero is zero, obtain a maximum value at (B/2, 1), since the sine of B/2 equals one, then decrease towards (B,0). Since the graph of y = sin(x) will repeat as we complete more than one revolution, we can now obtain the complete graph of the real valued function y = sin(x). Answers to Exercises I. a) 2B/3 b) 7B/4 c) -7B/3 d) -120 e) 540 f) 259.0o o o II. a) 2 b) /2 c) Does not exist III. a) -/2/2 b) 1 c) /3/2 d) -/2/2 e) -/3/2 f) -½ IV. a) .8059 b) .2225 c) -1.8871 V. a) The ladder is 20m long and reaches up approximately 17.32m. b) The bear is approximately 69.28m away. c) The height of the building and the distance of the observer are both about 20.49m. VI. a) B/6 + 2kB and 11B/6 + 2kB b) 5B/6 + 2kB and 7B/6 + 2kB c) 3B/2 + 2kB d) 7B/6 + 2kB and 11B/6 + 2kB e) 2B/3 + 2kB and 4B/3 + 2kB f) B/4 + kB VII. d) kB and B/2 + 2kB e) all x-values VIII. a) b)