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Learning Objectives

  • Identify and use the tangent ratio in a right triangle.
  • Understand tangent ratios in special right triangles.

The Tangent Ratio

The first ratio to examine when studying right triangles is the tangent.

The tangent of an angle is the ratio of the length of the opposite side to the length of the adjacent side. The hypotenuse is not involved in the tangent at all.

Recall that a ratio is the same as a fraction, so the tangent is the fraction of the opposite side over the adjacent side.

This means that tangent is: the ____________________ side divided by the ____________________ side.

Be sure when you find a tangent that you find the opposite and adjacent sides relative to the angle in question.

You must be careful that the opposite side is across from the angle you are taking the tangent of, and the adjacent side is next to that same angle!

For an acute angle measuring \theta, we define:

\tan \theta = \frac{opposite}{adjacent}

Like always, be sure to reduce the fraction in your final answer!

Reading Check:

1. Fill in the blank: Another word for ratio is _______________________.

2. Which side of a right triangle is NOT used in the tangent ratio?

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Example 1

What are the tangents of \angle X and \angle Y in the triangle below?

To find these ratios, first identify the sides opposite and adjacent to each angle:

For angle X:

The side opposite angle X is the segment ___________, which is _______ cm long.

The side adjacent to angle X is the segment ___________, which is _______ cm long.

For angle Y:

The side opposite angle Y is the segment ___________, which is _______ cm long.

The side adjacent to angle Y is the segment ___________, which is _______ cm long.

\tan \angle X & =\frac{opposite}{adjacent}=\frac{5}{12}\\\tan \angle Y & = \frac{opposite}{adjacent}=\frac{12}{5}

So, the tangent of\angle X is \frac{5}{12} and the tangent of \angle Y is \frac{12}{5}.

Notice that the tangent is different for different angles because of which sides are opposite and which sides are adjacent to each angle in the triangle.

It is common to write \tan X instead of \tan\angle X. In this text we will use both notations.

Reading Check:

On the blank triangle above,

1. Label the right angle.

2. Label the triangle \Delta CAT, where the right angle is at angle A.

3. Label the hypotenuse.

4. On the side opposite angle C, label “opposite \angle C

5. On the side adjacent to angle C, label “adjacent \angle C

6. On the side opposite angle T, label “opposite \angle T

7. On the side adjacent to angle T, label “opposite \angle T

(Note: some sides will have more than one label!)

Tangents of Special Right Triangles

It may help you to learn some of the most common values for tangent ratios. The list below shows you values for angles in special right triangles.

Tangent 30^\circ: \frac{1}{\sqrt{3}}\cdot \frac {\sqrt{3}}{\sqrt {3}} = \frac {\sqrt{3}}{3} this is approximately equal to (or \approx) 0.577

Tangent 45^\circ: \frac{1}{1} =  1

Tangent 60^\circ : \frac{\sqrt{3}}{1} = \sqrt{3} \approx 1.732

Notice that you can derive these ratios from the 30^\circ - 60^\circ - 90^\circ special right triangle. We will see this on the following page.

The triangle below is labeled with the side lengths that correspond to a 30^\circ - 60^\circ - 90^\circ triangle, the shortest side being x (as you saw in Unit 3):

If we start with the 30^\circ angle at the top of the triangle: the opposite side is x and the adjacent side is x\sqrt{3}.

Therefore, tangent of 30^\circ = \frac{x}{x\sqrt{3}} = \frac{1}{\sqrt{3}} or \frac{\sqrt{3}}{3}

Next we will use the 60^\circ angle on the right of the triangle: the opposite side is x\sqrt{3} and the adjacent side is x.

Therefore, tangent of 60^\circ  = \frac {x\sqrt{3}}{x}  =\frac{\sqrt{3}}{1} or \sqrt{3}

In order to figure out tangent of the 45^\circ, we must consider a right isosceles triangle, which has one angle that is 90^\circ and the other two that are the same measure (remember the definition of an isosceles triangle!)

This means that a right isosceles triangle has one angle that is _______ and the other two that equal _______.

Likewise, both legs of the isosceles triangle are the same length! When you take the tangent of the opposite leg over the adjacent leg, the value is 1.

You can use these ratios to identify angles in a triangle. Work backwards from the ratio. If the ratio equals one of these values, you can identify the measurement of the angle.

Example 2

What ism\angle{J} in the triangle below?

Find the tangent of \angle{J} and compare it to the values of tangent for special right triangles.

\tan \ J & = \frac {opposite}{adjacent}\\& = \frac{5}{5}\\& = 1

So, the tangent of \angle {J} is 1. If you look in the list of tangent values, you can see that an angle that measures 45^\circ has a tangent of 1. So, m\angle {J}=45^\circ.

(You may also notice that the triangle in this example is a right isosceles triangle, so the measure of both angles J and K must be 45^\circ.)

Example 3

What is m\angle {Z} in the triangle below?

Find the tangent of \angle{Z} and compare it to the values of tangent for special right triangles.

tan \ Z & = \frac {opposite}{adjacent}\\& = \frac{5.2}{3}\\& = 1.7{\overline{3}}

So, the tangent of \angle {Z} is about 1.73. If you look at the values of tangent for special triangles a few pages back, you can see that an angle that measures 60^\circ has a tangent of 1.732.So,m \angle Z \approx 60^\circ.

Another interesting thing to notice in this example is that \Delta XYZ is a 30^\circ - 60^\circ - 90^\circ triangle. You will remember from Unit 3 that this means that the sides of the triangle have a special relationship as well. You can use this fact to see that:

XY = 5.2 \approx 3\sqrt{3}

and if XZ = 3 then of course XY = 3\sqrt{3}!

Reading Check (Challenge):

Below is a 45^\circ - 45^\circ - 90^\circ triangle.

1. Which side length is the hypotenuse?

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2. Show your work to find the tangent of a 45^\circ angle. (Notice that it does NOT matter which 45^\circ angle you choose!)

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Date Created:

Feb 23, 2012

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Jun 14, 2014
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