Alternate Interior Angles: Definition, Theorem & Examples

In this lesson, you will learn how to identify alternate interior angles and how to use the theorem to find missing angles and to solve everyday geometry problems.
An angle is formed when two rays, a line with one endpoint, meet at one point called a vertex. The angle is formed by the distance between the two rays. Angles in geometry are often referred to using the angle symbol so angle A would be written as angle A.
A transversal line is a line that crosses or passes through two other lines. Sometimes, the two other lines are parallel, and the transversal passes through both lines at the same angle. The two other lines don’t necessarily have to be parallel in order for a transversal to cross them.
A straight angle, also called a flat angle, is formed by a straight line. The measure of this angle is 180 degrees. A straight angle can also be formed by two or more angles that sum to 180 degrees. Here, angle 1 + angle 2 = 180.
Transversal Line
Parallel lines are two lines on a two-dimensional plane that never meet or cross. When a transversal passes through parallel lines, there are special properties about the angles that are formed that do not occur when the lines are not parallel. Notice the arrows on lines m and n towards the left. These arrows indicate that lines m and n are parallel.
parallel lines
Alternate interior angles are formed when a transversal passes through two lines. The angles that are formed on opposite sides of the transversal and inside the two lines are alternate interior angles. Notice the pairs of blue and pink angles.
Alternate Interior Angles
These pairs are alternate interior angles.
Another way to think about alternate interior angles is by using the z-pattern. Notice that the pair of alternate interior angles makes a Z.
z pattern
In this window pane, angle a and angle b are alternate interior angles because they are on opposite sides of the transversal but inside the parallel lines.
Window Pane
Alternate Interior Angles Theorem
The Alternate Interior Angles theorem states, if two parallel lines are cut by a transversal, then the pairs of alternate interior angles are congruent.
A theorem is a proven statement or an accepted idea that has been shown to be true. The converse of this theorem, which is basically the opposite, is also a proven statement: if two lines are cut by a transversal and the alternate interior angles are congruent, then the lines are parallel.
These theorems can be used to solve problems in geometry and to find missing information. This diagram shows which pairs of angles are equal and alternate interior. Notice that the lines are parallel.
alternate interior angles
1. Use the Alternate Interior Angle theorem to find the measure of angle x, angle y, and angle z. Assume the lines are parallel.
alternate interior angles example
First, we need to identify a pair of alternate interior angles. Angle y and 58 are on opposite sides of the transversal and inside the parallel lines, so they must be alternate interior. Since the lines are parallel, angle y = 58.
Next, notice that angle x and 58 form a straight angle. Since a straight angle measures 180 degrees, angle x + 58 = 180 and 180 – 58 = angle x, so angle x = 122.
Finally, angle x and angle z are alternate interior angles, and we know that alternate interior angles are equal. So, angle x = 122 then angle z = 122.
2. Use the Alternate Interior Angles theorem to first find x and y and then to find the measures of the angles. Assume the lines are parallel.
ala example 2
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