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JEE Main 2026 Preparation: Question Papers, Solutions, Mock Tests & Strategy Unacademy » JEE Study Material » Mathematics » Alternate Interior Angles

Alternate Interior Angles

All you need to know about alternate interior angles, it's properties and transversal lines at Unacademy

Table of Content
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Introduction:

Definition of Interior Angle

The angles formed on the opposite sides of the transversal can be called alternate interior angles. In other words, eight angles are generated when two parallel lines are intersected by a transversal. The alternate interior angles are the angles that are on the inner side of the parallel lines but on the opposite sides of the transversal.

When a transversal crosses two parallel lines, the pair of angles formed on the inner side of the parallel lines but opposing sides of the transversal is referred to as alternate interior angles. These angles are always in the same proportion. This can also be interpreted differently. The alternate angles can be used to find the given lines are parallel or not. The given lines that are crossed by a transversal are said to be parallel if these angles are equal. When a transversal connects two coplanar lines, alternate interior angles are generated. They are on the inner side of the parallel lines but on the transversal lines’ opposing sides. The transversal runs between two parallel lines that are coplanar at different locations. These angles indicate whether or not the two supplied lines are parallel to one another. When these angles are equivalent, the lines that the transversal crosses are parallel.

Alternate Interior Angle

The alternate interior angles can be seen in the diagram below. l and m are two parallel lines that are intersected by a transversal line t.

The pairs of alternate interior angles in the above figure, according to the alternate interior angles theorem, are:

∠4 and ∠5

∠3 and ∠6

Let’s take a quick look at alternate external angles and how they differ from alternate interior perspectives. 

Alternate Exterior Angle

Angles with distinct vertices, lying on opposite sides of the transversal, and being external to the lines are known as alternate exterior angles. The alternate exterior angles created when a transversal intersects two parallel lines are always equal. The pairs of alternate exterior angles in the same figure are 1 & 8 and 2 & 7.

The Converse of Alternate Interior Angles Theorem

If a transversal intersects by two lines such that the alternate interior angles are the same or equal, the two lines are said to be parallel, according to the converse of the alternate interior angles theorem.

What Is the Best Way to Find Alternate Interior Angles?

According to the alternate interior angles theorem, the alternate interior angles of two parallel lines are equal. We take advantage of this fact to come up with new interior angles. 

Let us take an example to understand this clearly, in the map below, the road designated Sixth Avenue runs perpendicular to the parallel streets of 1st Street and 2nd Street. Another route, Maple Avenue, forms a 40-degree angle with 2nd Street. calculate the angle x’s measurement?

Let us Calculate the above Problem, If the two streets are parallel and Maple Avenue is regarded as the transversal, then x and 40° are the alternate interior angles, according to the alternate interior angles theorem. As a result, both angles are equal. As a result, x = 40°.

Important Reminders:

  • Each alternate interior angle pair is the same.
  • Each pair of co-interior angles adds to give 180 degrees.
  • Each corresponding angle pair is the same.
  • Each alternate pair of external angles is the same.
  • Alternate Interior Angles are two angles on the inner side of each of those two lines but separate sides of the transversal.
  • When a transversal travels through two lines, it creates alternate interior angles. Alternate interior angles are generated on opposite sides of the transversal and inside the two lines.
faq

Frequently asked questions

Get answers to the most common queries related to the IIT JEE Examination Preparation.

In geometry, what are alternate interior angles?

Ans : Solution: To establish alternate internal angles...Read full

What Is The Best Way To Solve Alternate Interior Angles?

Ans : Solution: If two parallel lines are intersected by a transversal, then alternate inter...Read full

What does the Alternate Interior Angles Theorem's inverse look like?

Ans : Solution: If a transversal intersects two lines such that the alternate interior angle...Read full

What Is the Distinction Between Corresponding Angles and Alternate Angles?

Ans : Solution: Corresponding angles are two angles on the same side of the transversal, one...Read full

What Do Alternate Interior and Exterior Angles Mean and also state that Alternate Interior Angles Are Complementary or Congruent?

Ans: Solution: Alternate interior angles are those with distinct vertices that lie on opposite sides of the t...Read full

Ans : Solution: To establish alternate internal angles in geometry, we must look at the following characteristics:

In terms of measure, the alternate internal angles are equivalent.

They are positioned on opposite sides of the transversal.

They are located in the middle of the two lines.

In other words, the angles that are on the inner side of the parallel lines but the transversal lines’ opposing sides.

Ans : Solution: If two parallel lines are intersected by a transversal, then alternate interior angles are identical in measure, according to the alternate interior angles theorem. If we know the measure of the corresponding alternate interior angle, we may use this theorem to find the measure of the alternate interior angle.

Ans : Solution: If a transversal intersects two lines such that the alternate interior angles are equal, the two lines are said to be parallel, according to the converse of the alternate interior angles theorem.

Ans : Solution: Corresponding angles are two angles on the same side of the transversal, one of which is interior and the other of which is exterior. The alternate angles are two angles on the transversal that are on opposite sides.

Ans: Solution: Alternate interior angles are those with distinct vertices that lie on opposite sides of the transversal and are in the middle of the two lines’ interior. On the other hand, alternate external angles are those with different vertices and are located on the opposite sides of the transversal, but on the outside side of the two lines. The alternate interior angles of two parallel lines are congruent according to the alternate interior angles theorem.

 

 

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