In geometry, alternate interior angles occur when parallel lines are cut by a transversal. These lines form angles that are congruent and are often referred to as alternate interior angles (AIANs). The angles m3 and m6 are examples of alternate interior angles. In this article, we’ll learn about their properties, how to determine whether or not they are parallel, and how to find missing angles.

## Congruent

When two parallel lines intersect, the interior angles are congruent. A congruent angle is one that has the same measure and proportion as an opposite angle. A parallel line may also be congruent if the exterior and interior angles are on opposite sides of the same line. Another example of a congruent angle is an interior angle formed by two intersecting diagonals.

Two congruent angles can be constructed step-by-step. First, construct two horizontal lines (or arcs). You can use any length of the lines and then place the compass tip at the intersection of the two arcs.

## Determines whether two lines are parallel

Two lines are parallel if their slopes are equal. Therefore, two lines that are parallel will have the same slope but different y-intercepts. This means that the lines will intersect at many points but never exactly meet. Let’s look at two examples to illustrate how to determine whether two lines are parallel. These examples are shown in Figure 1. After you’ve determined whether two lines are parallel, you’ll need to determine whether they have the same slope.

A parallel line will never intersect, but it will have the same slope as the other line. A parallel line will have the same height, and the slopes of the two lines must be the same. In addition, a parallel line must lie in the same plane. The y-intercepts of the two lines should be different, but the x-intercepts should be the same.

## Shows the existence of parallel lines

A line is parallel to another line if it never intersects itself. This is called a parallel line, and it is marked by a symbol. A line AB is parallel to a line XY, and so it is written as AB XY. This is not true if the two lines intersect on different planes.

If two lines have the same slope and steepness, then they are parallel. A line that intersects two parallel lines must be transversal to them.

## Finds missing angles

In eighth grade, students should learn how to apply the Triangle Angle-Sum Theorem to find missing interior angles in triangles. These math worksheet models the concept and challenge students to apply it to real-world situations. It includes two pages of practice problems. One of them is titled Interior Angles in Triangles – Solve for Variable.