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Home , Right angle, Right triangle, Special right triangle

6.1 Basic Right

Trigonometry

MEASURING IN

First, let’s introduce the units you will be using to measure angles, radians.

A is a unit of measurement defined as the at the center of the made when the arc equals the radius.

If this definition sounds abstract we define the radian pictorially below. Assuming the radius drawn below equals the between the x-axis and where the radius intersects the circle, then the angle Θ is 1 radian. Note that 1 radian is approximately 57°.

Θ ≈ 57°

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Many people are more familiar with a measurement of an angle. Below is a quick formula for converting between degrees and radians. You may use this in order to gain a more intuitive understanding of the magnitude of a given radian measurement, but for most classes at R.I.T. you will be using radians in computation exclusively. 휋 푟푎푑푖푎푛푠 = 푑푒푔푟푒푒푠 180

Now consider the pictured below with sides a,b,c and angles A,B,C. We will be referencing this generic representation of a right triangle throughout the packet.

B

c a

A

C b

BASIC FACTS AND DEFINITIONS

휋 1. : angle measuring radians (example: angle C above) 2 2. Straight angle: angle measuring π radians 휋 3. Acute angle: angle measuring between 0 and radians (examples: angles A and B 2 above) 휋 4. Obtuse angle: angle measuring between and π radians 2 휋 5. Complementary angles: Two angles whose sum is radians. Note that A and B are 2 휋 complementary angles since C = radians and all have a sum of π radians 2 between the three angles. 6. Supplementary angles: two angles whose sum is π radians 2

휋 7. Right triangle: a triangle with a right angle (an angle of radians) 2 8. : a triangle with exactly two sides of equal length 9. : a triangle with all three sides of equal length 10. : side opposite the right angle, side c in the diagram above 11. : 푐ퟐ = 푎2 + 푏2

Example 1: A right triangle has a hypotenuse length of 5 inches. Additionally, one side of the triangle measures 4 inches. What is the length of the other side?

Solution:

푐ퟐ = 푎2 + 푏2

5ퟐ = 42 + 푏2

25 = 16 + 푏2

9 = 푏2

b = 3 inches

휋 Example 2: In the right triangle pictured above, if 퐴 = radians, what is the measure of angle 6 B?

Solution:

The two acute angles in a right triangle are complementary, so:

휋 A + B = 2

휋 휋 + B = 6 2

휋 B = 3

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SIMILIAR TRIANGLES

Two triangles are said to be similar if the angles of one triangle are equal to the corresponding 휋 angles of the other. That is, we say triangles ABC and EFG are similar if A = E, B = F, and C = G = 2 radians and we write 퐴퐵퐶~퐸퐹퐺. Further, the ratio of corresponding sides are equal, that is;

퐴퐶 퐴퐵 퐵퐶 = = 퐸퐺 퐸퐹 퐹퐺 B F

A

C E G Example 3: Let AE = 50 meters, EF = 22 meters and AB = 100 meters. Find the length of side BC.

Notice that ABC and AEF are similar since corresponding angles are equal. (There is a right angle at both F and C, angle A is the same in both triangles and angle B equals angle E).

퐴퐸 퐸퐹 50 22 Solution: = thus = 퐴퐵 퐵퐶 100 퐵퐶

So 50(BC) = (22)(100)

BC = 44 meters

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THE SIX TRIGONOMETRIC RATIOS FOR ACCUTE ANGLES

The trigonometric ratios give us a way of relating the angles to the ratios of the sides of a right triangle. These ratios are used pervasively in both physics and engineering (especially the introductory phyiscs sequence at RIT). Below we define the six and then to some examples in which they must be applied.

Example 4 In the right triangle ABC, a = 1 and b = 3. Determine the six trigonometric ratios for angle B.

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SPECIAL TRIANGLES

There are certain right triangles with characteristic side angles that appear so often in problems 휋 휋 we make note of two of them here. The first has accute angles of and , note that all such 6 3 traingles are similar. Thus for some length x, the side and trigonometric ratios of such a triangle are shown below. 휋 1 푠푖푛 = 휋 √3 6 2 푠푖푛 = B 3 2

휋 √3 휋 1 휋 c = 2x 푐표푠 = 푐표푠 = 6 2 3 2 6 a = √3x 휋 휋 √3 푡푎푛 = √3 푡푎푛 = 3 휋⁄3 6 3 A 휋 휋 2√3 C b = x 푐푠푐 = 2 푐푠푐 = 6 3 3 휋 2√3 휋 푠푒푐 = 푠푒푐 = 2 6 3 3

휋 휋 √3 푐표푡 = √3 푐표푡 = 6 3 3

휋 The second we will look at has both acute angles to be . Then for some 4 length x, the side lengths and trigonometric ratios of such a triangle are shown below.

B 휋 √2 푠푖푛 = 4 2 c = √2x 휋 휋 √2 4 푐표푠 = a = x 4 2

휋⁄4 휋 푡푎푛 = 1 A 4 C b = x 휋 푐푠푐 = √2 4

휋 푠푒푐 = √2 4 휋 푐표푡 = 1 6 4

Example 5:

휋 휋 휋 If the hypotenuse of a - - triangle is 10 meters, what are the lengths of the triangles legs? 6 3 2

Solution:

If 10 = 2x, then x = 5 meters and √3푥 = √3(5) = ퟓ√ퟑ meters

Problems

1. In right triangle ABC, if side c = 39 (hypotenuse) and side b = 36, find a.

2. Given the lengths defined in the right triangle pictured below, find the length of side AC.

3. Evauluate: a. sin E = ______b. tan E = ______c. cos F = ______d. sec F = ______

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4. Evaluate: 휋 sin = ______6 휋 sec = ______3 휋 csc = ______4 휋 tan = ______3

휋 tan = ______4 휋 cot = ______6

5. Evaluate: a. tan A =______b. csc B = ______

c. cot A = ______d. sec B = ______

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Answers

1. a = 15 (pythagorean theorem) 2. AC = 18 (properties of similar triangles) 3. 12 a. 13 12 b. 5 12 c. 13 13 d. 12 4. 1 a. 2 b. 2 c. √2 d. √3 e. 1 f. √3 5. 4 a. 3 5 b. 3 3 c. 4 5 d. 4

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