What if you were told the tangent of is 0.6494? How could you find the measure of ? After completing this Concept, you'll be able to find angle measures by using the inverse trigonometric ratios.

### Watch This

CK-12 Foundation: The Inverse Trigonometric Ratios

James Sousa: Introduction to Inverse Trigonometric Functions

### Guidance

In mathematics, the word inverse means “undo.” For example, addition and subtraction are inverses of each other because one undoes the other. When we use the inverse trigonometric ratios, we can find acute angle measures as long as we are given two sides.

**
Inverse Tangent:
**
Labeled
, the “-1” means inverse.

and .

**
Inverse Sine:
**
Labeled
.

and .

**
Inverse Cosine:
**
Labeled
.

and .

In most problems, to find the measure of the angles you will need to use your calculator. On most scientific and graphing calculators, the buttons look like , and . You might also have to hit a shift or button to access these functions.

Now that you know both the trig ratios and the inverse trig ratios you can
**
solve
**
a right triangle. To solve a right triangle, you need to find all sides and angles in it. You will usually use sine, cosine, or tangent; inverse sine, inverse cosine, or inverse tangent; or the Pythagorean Theorem.

#### Example A

Use the sides of the triangle and your calculator to find the value of . Round your answer to the nearest tenth of a degree.

In reference to
, we are given the
*
opposite
*
leg and the
*
adjacent
*
leg. This means we should use the
*
tangent
*
ratio.

. So, . Now, use your calculator.

If you are using a TI-83 or 84, the keystrokes would be:
[TAN]
**
[ENTER]
**
and the screen looks like:

#### Example B

is an acute angle in a right triangle. Find to the nearest tenth of a degree.

a)

b)

c)

**
Answers:
**

a)

b)

c)

#### Example C

Solve the right triangle.

To solve this right triangle, we need to find and . Use only the values you are given.

: Use the Pythagorean Theorem.

: Use the inverse sine ratio.

: Use the inverse cosine ratio.

CK-12 Foundation: The Inverse Trigonometric Ratios

### Guided Practice

1. Solve the right triangle.

2. Solve the right triangle.

3. When would you use and when would you use ?

**
Answers:
**

1. To solve this right triangle, we need to find and .

: Use the sine ratio.

: Use the tangent ratio.

: Use the Triangle Sum Theorem

2. The two acute angles are congruent, making them both . This is a 45-45-90 triangle. You can use the trigonometric ratios or the special right triangle ratios.

Trigonometric Ratios

45-45-90 Triangle Ratios

3. You would use when you are given an angle and you are solving for a missing side. You would use when you are given sides and you are solving for a missing angle.

### Practice

Use your calculator to find to the nearest tenth of a degree.

Let be an acute angle in a right triangle. Find to the nearest tenth of a degree.

Solving the following right triangles. Find all missing sides and angles. Round any decimal answers to the nearest tenth.