8.9: Inverse Trigonometric Ratios
What if you were told that the longest escalator in North America is at the Wheaton Metro Station in Maryland and is 230 feet long and is 115 ft high? What is the angle of elevation,
Watch This
CK12 Foundation: Chapter8InverseTrigonometricRatiosA
James Sousa: Introduction to Inverse Trigonometric Functions
Guidance
The word inverse is probably familiar to you. In mathematics, once you learn how to do an operation, you also learn how to “undo” it. For example, you may remember that addition and subtraction are considered inverse operations. Multiplication and division are also inverse operations. In algebra you used inverse operations to solve equations and inequalities. When we apply the word inverse to the trigonometric ratios, we can find the acute angle measures within a right triangle. Normally, if you are given an angle and a side of a right triangle, you can find the other two sides, using sine, cosine or tangent. With the inverse trig ratios, you can find the angle measure, given two sides.
Inverse Tangent: If you know the opposite side and adjacent side of an angle in a right triangle, you can use inverse tangent to find the measure of the angle. Inverse tangent is also called arctangent and is labeled
Inverse Sine: If you know the opposite side of an angle and the hypotenuse in a right triangle, you can use inverse sine to find the measure of the angle. Inverse sine is also called arcsine and is labeled
Inverse Cosine: If you know the adjacent side of an angle and the hypotenuse in a right triangle, you can use inverse cosine to find the measure of the angle. Inverse cosine is also called arccosine and is labeled
Using the triangle below, the inverse trigonometric ratios look like this:
In order to actually find the measure of the angles, you will need you use your calculator. On most scientific and graphing calculators, the buttons look like
Now that we know how to use inverse trigonometric ratios to find the measure of the acute angles in a right triangle, we can solve right triangles. To solve a right triangle, you would need to find all sides and angles in a right triangle, using any method. When solving a right triangle, you could use sine, cosine or tangent, inverse sine, inverse cosine, or inverse tangent, or the Pythagorean Theorem. Remember when solving right triangles to only use the values that you are given.
Example A
Use the sides of the triangle and your calculator to find the value of
In reference to
If you are using a TI83 or 84, the keystrokes would be:
So,
Example B
a)
b)
c)
Solutions:
a)
b)
c)
Example C
Solve the right triangle.
To solve this right triangle, we need to find
Watch this video for help with the Examples above.
CK12 Foundation: Chapter8InverseTrigonometricRatiosB
Concept Problem Revisited
To find the escalator’s angle of elevation, we need to use the inverse sine ratio.
Vocabulary
Trigonometry is the study of the relationships between the sides and angles of right triangles. The legs are called adjacent or opposite depending on which acute angle is being used. The three trigonometric (or trig) ratios are sine, cosine, and tangent. The inverse trig ratios,
Guided Practice
1. Solve the right triangle.
2. Solve the right triangle.
Answers:
1. To solve this right triangle, we need to find
2. Even though, there are no angle measures given, we know that the two acute angles are congruent, making them both
Trigonometric Ratios
454590 Triangle Ratios
Practice
Use your calculator to find
Let

sinA=0.5684 
cosA=0.1234 
tanA=2.78
Solve the following right triangles. Find all missing sides and angles.
 Writing Explain when to use a trigonometric ratio to find a side length of a right triangle and when to use the Pythagorean Theorem.
Adjacent Angles
Two angles are adjacent if they share a side and vertex. The word 'adjacent' means 'beside' or 'nextto'.Conic
Conic sections are those curves that can be created by the intersection of a double cone and a plane. They include circles, ellipses, parabolas, and hyperbolas.cosine
The cosine of an angle in a right triangle is a value found by dividing the length of the side adjacent the given angle by the length of the hypotenuse.Hypotenuse
The hypotenuse of a right triangle is the longest side of the right triangle. It is across from the right angle.Pythagorean Theorem
The Pythagorean Theorem is a mathematical relationship between the sides of a right triangle, given by , where and are legs of the triangle and is the hypotenuse of the triangle.sine
The sine of an angle in a right triangle is a value found by dividing the length of the side opposite the given angle by the length of the hypotenuse.Slope
Slope is a measure of the steepness of a line. A line can have positive, negative, zero (horizontal), or undefined (vertical) slope. The slope of a line can be found by calculating “rise over run” or “the change in the over the change in the .” The symbol for slope isTangent
The tangent of an angle in a right triangle is a value found by dividing the length of the side opposite the given angle by the length of the side adjacent to the given angle.Image Attributions
Description
Learning Objectives
Here you'll learn how to use inverse trigonometric ratios to solve for missing angles in right triangles.