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**Right Triangle Trigonometry**

MAT 105 SP09 Right Triangle Trigonometry

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**The sides of a right triangle**

MAT 105 SP09 The sides of a right triangle Take a look at the right triangle, with an acute angle, , in the figure below. Notice how the three sides are labeled in reference to . Side adjacent to Side opposite Hypotenuse In this section, we will be studying special ratios of the sides of a right triangle, with respect to angle, .

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**These ratios are better known as our six basic trig functions:**

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**Definitions of the Six Trigonometric Functions**

MAT 105 SP09 Definitions of the Six Trigonometric Functions

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**“SOH CAH TOA” Definitions of the Six Trigonometric Functions**

MAT 105 SP09 Definitions of the Six Trigonometric Functions To remember the definitions of sine, cosine and tangent, we use the acronym : “SOH CAH TOA”

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MAT 105 SP09 Example Find the exact value of the six trig functions of in the triangle below: First find the length of the hypotenuse using the Pythagorean Theorem. 5 9

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MAT 105 SP09 Example Given that is an acute angle and , find the exact value of the five remaining trig functions of .

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MAT 105 SP09 Example Find the value of sin given cot = 0.387, where is an acute angle. (Divide ratio and give answer to three significant digits.)

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**Special Right Triangles**

MAT 105 SP09 Special Right Triangles The 45º- 45º- 90º Triangle Ratio of the sides: Find the exact values & decimal approximations (to 3 sig digits) of the six trig functions for 45 sin 45 = ≈ csc 45 = ≈ cos 45 = ≈ sec 45 = ≈ tan 45 = cot 45 = 1 45º

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**Special Right Triangles**

MAT 105 SP09 Special Right Triangles The 30º- 60º- 90º Triangle Ratio of the sides: Find the exact values & decimal approximations (to 3 sig digits) of the six trig functions for 30 sin 30 = csc 30 = cos 30 = ≈ sec 30 = ≈ tan 30 = ≈ cot 30 = ≈ 1 60º 30º 2

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**Special Right Triangles**

MAT 105 SP09 Special Right Triangles The 30º- 60º- 90º Triangle Ratio of the sides: Find the exact values & decimal approximations (to 3 sig digits) of the six trig functions for 60 sin 60 = ≈ csc 60 = ≈ cos 60 = sec 60 = tan 60 = ≈ cot 60 = ≈ 1 60º 30º 2

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**Using the Calculator to Evaluate Trig Functions**

To evaluate trig functions of acute angles other than 30, 45, and 60, you will use the calculator. Your calculator has keys marked , , and . **Make sure the MODE is set to the correct unit of angle measure. (Degree vs. Radian) Example: Find to two decimal places. 13

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**Angles and Accuracy of Trigonometric Functions**

MAT 105 SP09 Angles and Accuracy of Trigonometric Functions Measurement of Angle to Nearest Accuracy of Trig Function 1° 2 significant digits 0. 1° or 10' 3 significant digits 0. 01° or 1' 4 significant digits

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**X Using the Calculator to Evaluate Trig Functions**

MAT 105 SP09 Using the Calculator to Evaluate Trig Functions To find the values of the remaining three functions (cosecant, secant, and tangent), use the reciprocal identities. For reciprocal functions, you may use the button, but DO NOT USE THE INVERSE FUNCTIONS (e.g. SIN-1 )! Example: 1. Find Find (to 3 significant dig) (to 4 significant dig) X

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**The Inverse Trigonometric Functions**

MAT 105 SP09 The Inverse Trigonometric Functions The inverse trig functions give the measure of the angle if we know the value of the function. Notation: The inverse sine function is denoted as sin-1x or arcsin x. It means “the angle whose sine is x”. The inverse cosine function is denoted as cos-1x or arccos x. It means “the angle whose cosine is x”. The inverse tangent function is denoted as tan-1x or arctan x. It means “the angle whose tangent is x”.

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**Example of Inverse Trig Function**

For example, will yield the acute angle whose sine is . You can think of this as the related equation

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**Examples using common angles**

Evaluate the following inverse trig functions using the special triangles (you do not need a calculator): 1) )

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**Examples using the calculator**

MAT 105 SP09 Examples using the calculator Evaluate the following inverse trig functions using the calculator. Give answer in degrees. Round appropriately.

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**Examples using the calculator**

MAT 105 SP09 Examples using the calculator Evaluate the following inverse trig functions using the calculator. Give answer in degrees. Round appropriately.

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**Using Trig Ratios to Find Missing Parts of Right Triangles**

MAT 105 SP09 Using Trig Ratios to Find Missing Parts of Right Triangles Example: Solve for y in the right triangle below: y 52º 9.6 Solution: Since you are looking for the side adjacent to 52º and are given the hypotenuse, you could use the _____________ function.

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**Solving Right Triangles**

To solve a right triangle is to find any missing angles and any missing sides. You will always be given 3 parts, and you will need to find 3 parts. The angles are labeled using capital letters A, B, & C. Use angle C to represent the right angle. Angles A and B represent the acute angles. The sides are labeled using lowercase letters a, b, & c. Each side is labeled with respect to its opposite angle.

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**Example Solve the right triangle with the indicated measures. Solution**

MAT 105 SP09 Example Solve the right triangle with the indicated measures. A= 40.7° C B b c a=8.20” Solution Answers:

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MAT 105 SP09 Example A C B b c=35.4 a=25.8 Answers:

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**Example 3. Find the altitude of the isosceles triangle below. 36°**

MAT 105 SP09 Example 3. Find the altitude of the isosceles triangle below. 36° 8.6 m

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MAT 105 SP09 Example 4. Solve the right triangle with Answers:

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**Angle of Elevation and Angle of Depression**

MAT 105 SP09 Angle of Elevation and Angle of Depression The angle of elevation for a point above a horizontal line is the angle formed by the horizontal line and the line of sight of the observer at that point. The angle of depression for a point below a horizontal line is the angle formed by the horizontal line and the line of sight of the observer at that point. Horizontal line Angle of depression Angle of elevation Horizontal line

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MAT 105 SP09 Example A guy wire of length 108 meters runs from the top of an antenna to the ground. If the angle of elevation of the top of the antenna, sighting along the guy wire, is 42.3° then what is the height of the antenna? Give answer to three significant digits. Solution 108 m 42.3° h

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MAT 105 SP09

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Chapter 4 Review of the Trigonometric Functions

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