Alternate Interior Angles

In geometry, alternate interior angles are special pairs of angles formed when a line, called a transversal, intersects two other lines. They sit on opposite sides of the transversal and inside the two other lines. When the lines being intersected are parallel, these angles are equal in measure.

In this article, we will discuss the concept of Alternate Interior Angles in detail including its definition, example, and properties. We will also learn about the Alternate Interior Angle Theorem and its converse and their proof as well.

Alternate interior angles are pairs of angles that lie on opposite sides of a transversal line and inside a pair of lines but on opposite sides of the transversal. These angles are formed when a transversal intersects two parallel lines.

The key property of alternate interior angles is that they are equal in measure. In other words, if two parallel lines are intersected by a transversal, then the pairs of alternate interior angles are congruent.

Alternate Interior Angles Definition

The angles created on opposing sides of the transversal are known as alternate internal angles.

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

Alternate Interior Angles Examples

Let’s consider a pair of parallel line and a transversal as shown in the figure below:

The inner angles are angles ∠3, ∠4, ∠5, and ∠6. Out of these ∠3 & ∠6 and ∠4 & ∠5 are the examples of Alternate Interior Angles.

Note: Angles ∠3 and ∠5, ∠4 and ∠6, ∠1 and ∠7, and ∠2 and ∠8 are Alternates Angles.

There are various properties of Alternate Interior Angles, some of these properties are:

• Alternate Interior angles that are opposite one another are congruent (measure the same).
• Consecutive interior angles are alternate interior angles on the same side of the transversal line.
• The total of the angles created on the same side of the transversal that are within the two parallel lines is always 180°.
• Alternate interior angles do not have any special qualities in the case of non-parallel lines.

Are Alternate Interior Angles Congruent?

Yes, alternate interior angles are congruent when the lines intersected by the transversal are parallel.

For example, if line AB is parallel to line DE , and a transversal AC cuts across them, then the alternate interior angles formed, such as angles CAD and DAE , are congruent.

Excercise: Draw the diagram for above mentioned example.

According to the theorem, “a transversal crosses the set of parallel lines if the alternate interior angles are congruent.”

Proof of Alternate Interior Angles Theorem

Given: l || m

To Prove: ∠4 = ∠5 and ∠3 = ∠6

Proof: Assume that t is the transversal that crosses l and m. l and m are two parallel lines. See the illustration below.

We know from the characteristics of the parallel line that the corresponding angles and vertically opposed angles are identical when a transversal intersects any two parallel lines. Therefore,

From the properties of the parallel line, we know if transversal cuts any two parallel lines, the corresponding angles and vertically opposite angles are equal to each other. Therefore,

∠1 = ∠5 . . . (i) [Corresponding Angles]

∠1 = ∠4 . . . (ii) [Vertically Opposite Angles]

From Equation (i) and Equation (ii), we get

∠4 = ∠5 [Alternate interior angles]

Similarly,

∠3 = ∠6

So, it is proven.

Read More,

• Measuring Angles
• Alternate Exterior Angles
• Exterior Angle Theorem

If two lines are crossed by a transversal and the alternate interior angles are congruent, the lines must be parallel, according to the converse of Alternate Interior Angles. This theorem is useful for finding parallel lines based on the equality of their alternate interior angles, and it has practical applications in geometric reasoning and problem solving.

Proof of Converse of Alternate Interior Angles Theorem

Converse of Alternate Interior angles that are opposite each other. According to the theorem, if two lines and a transversal form alternating interior angles that are equivalent, the lines are parallel.

Given: Transversal EFGH connects lines AB and CD, resulting in a pair of equal alternative E.

To Prove: AB||CD

Proof:

∠AFG = ∠FGD

But ∠AFG = ∠EFB (Vertically opposite angles)

⇒∠EFB = ∠FGD (corresponding angles)

⇒AB || CD

The lines are parallel because their respective angles, AB and CD, are equal. As a result, the reverse of the alternative interior angles theorem is demonstrated.

Read More,

• Angles
• Types of Angles
• Lines and Angles

Example 1: Demonstrate that the interior angles (6x + 82)° and (8x – 88)° are congruent.

Answer:

Interior alternate angles are equal, As a result, 

(7x + 82) ° = (8x – 88) ° 

⇒ 7x + 82 = 8x – 88 

⇒ x = 170

In the original formulations, substitute x.

⇒ (7x + 82) ° = 1272°.

⇒ (8x – 88)° = 1272°

As a result, (7x + 82) ° = (8x – 88)°.

Example 2: P and Q are on opposite lanes. They planned to construct a path that would cut at an angle between the two pathways. What is the value of ∠Q if ∠P = 85°?

Answer:

Because the two pathways are joined by a transversal, the alternative angles should be identical, suggesting that ∠P equals ∠Q.

As a result, ∠Q = ∠P = 85°.

Example 3: If the alternate interior angles are indicated as (7x + 55)° and (8x – 95)° in a given set of two parallel lines cut by a transversal, calculate the value of x and the true value of the alternate interior angles using the alternate interior angles theorem.

Answer:

Alternate interior angles are (7x + 55)° and (8x – 95)°. L1 and L2 are parallel, hence the two angles are congruent. So,

(7x + 55)° = (8x – 95)°

⇒ 7x + 55 = 8x – 95 

⇒ -150 = -x 

As a result, x = 150°.

By changing the value of x with the provided angles, we can determine the exact value of the angles.

(7x + 55)° = [7(150) + 55]° = 1105°

(8x – 95)°= [8(150) − 95]° = 1105°

Because they are opposite inside angles, they have the same value.

Example 4: Mark the interior angles and write their values on the accompanying picture. In the diagram, ∠3 = 95°and ∠5 = 105°.

Solution:

The various inside angles in the preceding image are  ∠3, ∠4, ∠5, and ∠6. Only ∠3 and ∠5 are offered.

Because we already know that alternate inside angles are equivalent.

∠3 = ∠6 = 95°

∠4 = ∠5 = 105°

Define Alternate Interior Angles.

The angles created inside the two parallel lines when a transversal intersects them are known as the alternate interior angles, and they are equal to their alternate pairs.

Are Alternate Interior Angles Equal?

Yes, alternate internal angles are equal for parallel lines. When a transversal connects two parallel lines, the angles on opposing sides of the transversal have the same measure.

How Can Alternate Interior Angles Be Solved?

The alternate interior angles theorem states that if a transversal connects two parallel lines, the alternate interior angles are identical in size. If we know the measure of the corresponding alternate interior angle, we may use this theorem to determine the measure of the alternate interior angle.

Alternate Interior and Exterior Angles: What Is the Difference?

The angles that are between the interior of the two lines and have alternate interior angles are those with different vertices, which are on the alternate sides of the transversal. Alternate exterior angles, on the other hand, are those angles with distinct vertices that are located on the alternate sides of the transversal but are located on the outside of the two lines.

How Can Alternate Interior Angles Be Solved?

The alternate interior angles theorem states that if a transversal connects two parallel lines, the alternate interior angles are identical in size. If we know the measure of the corresponding alternate interior angle, we may use this theorem to determine the measure of the alternate interior angle.

Alternative Interior Angles: Supplementary or Congruent?

The alternate interior angles of two parallel lines are congruent or identical, according to the alternate interior angles theorem.

What are Same Side of Interior Angles?

Same-side interior angles are pairs of angles generated by two parallel lines and a transversal that are on the same side of the transversal. They add up to 180 degrees.