Alternate interior angles

🏆Practice angles in parallel lines

Alternate interior angles

Alternate interior angles are alternate angles located in the internal area between parallel lines. They are not on the same side of the transversal nor are they on the same level (floor) relative to the line.

Diagram showing corresponding interior angles in geometry with marked arcs in blue and connecting lines, featuring a quadrilateral structure and labeled by Tutorela.

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If one of two corresponding angles is a right angle, then the other angle will also be a right angle.

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Alternate interior angles

In this article, we will learn about alternate interior angles, how to identify them, as well as their characteristics.
First, we need to remember what alternate angles are in general:
Alternate angles
Alternate angles between parallel lines are equal.
They are called alternate angles because they:
• Are not on the same side of the transversal line
• Are not on the same "level" relative to the line

Here are alternate angles for example:

Diagram showing corresponding interior angles in geometry with marked arcs in blue and connecting lines, featuring a quadrilateral structure and labeled by Tutorela.

The two marked angles are not on the same level and not on the same side, therefore they are alternate angles.
In order to confirm the presence of an alternate interior angle, you must observe that:
There is an exterior part - outside the two parallel lines
As well as an interior part - between the two parallel lines.
Let's examine the illustration:

Diagram explaining corresponding exterior and interior angles with labeled sections: the exterior part in orange, the interior part in blue, and highlighted angles for geometry demonstration, designed for teaching angle relationships.

In the illustration, we can see that the two alternate angles located between the two parallel lines in the inner part are alternate interior angles. Let's examine another example of a pair of alternate interior angles:

Diagram showing corresponding interior angles in geometry with marked arcs in blue and connecting lines, featuring a quadrilateral structure and labeled by Tutorela.


Note that in this illustration as well, you can observe that the two alternate angles are located in the internal part between the two parallel lines, and therefore they are alternate interior angles.

Bonus tip!
Alternate angles located in the external part outside the two parallel lines are called exterior alternate angles.

And now let's practice!
Here are two parallel lines and a line intersecting them.
a. Determine whether the angles shown are alternate angles.
b. Determine whether they are also alternate interior angles.

Diagram showing corresponding interior angles in geometry with marked arcs in blue and connecting lines, featuring a quadrilateral structure and labeled by Tutorela.

Solution:
a. Yes, the angles in the figure are alternate angles. They are not on the same side of the transversal and not on the same level relative to the line.
b. Yes, the alternate angles in the figure are interior since they are located in the inner part between the two parallel lines.

Another exercise:
Two parallel lines and a line intersecting them are shown.
a. Determine whether the angles shown are alternate angles
b. Determine whether they are alternate interior angles.

Diagram explaining corresponding exterior and interior angles with labeled sections: the exterior part in orange, the interior part in blue, and highlighted angles for geometry demonstration, designed for teaching angle relationships.

Solution:
a. Yes, the angles in the figure are alternate angles. They are not on the same level relative to the line and not on the same side of the transversal.
b. No. The angles are located in the external part outside the two parallel lines, therefore they are alternate angles but not interior ones.

Another exercise:
Here are two parallel lines and a line that intersects them.
Find the size of angle AA
and determine whether angle AA and angle BB are alternate interior angles.
Given that: B=100B=100

Diagram illustrating the relationship between angles A and B formed by parallel lines and a transversal, with a mathematical equation overlay for calculating angles in polygons, presented by Tutorela.

Solution:
According to the given information as well as the provided figue , we can determine that angle AA and angle BB are alternate angles. They are located between two parallel lines, each on a different side of the transversal and not on the same level relative to the line.
Alternate angles are equal to each other, therefore if B=100B=100 we can conclude that angle A=100A=100

Additionally, we can also determine that the two angles are alternate interior angles because they are both located in the interior part between the two parallel lines.

Additional Exercise:

In all drawings, the two lines are parallel to each other.
a. Determine whether there are alternate interior angles in both drawings.
b. If in drawing 11 the marked angle QQ equals 130130, what is angle WW?
c. Determine true or false - only alternate exterior angles are equal to each other.

1.

Diagram demonstrating corresponding exterior angles in geometry, marked as 'w' and 'q' in blue, within a quadrilateral structure for educational purposes.

2.

Diagram showing corresponding interior angles in geometry with marked arcs in blue and connecting lines, featuring a quadrilateral structure and labeled by Tutorela.

Solution:
a. No, only in the second drawing the two angles are alternate interior angles given that they are located in the inner part of the lines.
In the first drawing, the two angles are alternate exterior angles since they are located in the outer part of the lines.

b. The two angles marked in the drawing 11 are alternate angles and therefore they are equal.
From this we can conclude that angle WW is also equal to 130130.

C. Incorrect – exterior alternate angles are also equal to each other.

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Examples with solutions for Angles in Parallel Lines

Exercise #1

If one of two corresponding angles is a right angle, then the other angle will also be a right angle.

Video Solution

Step-by-Step Solution

To solve this problem, consider the following explanation:

When dealing with the concept of corresponding angles, we are typically considering two parallel lines cut by a transversal. The property of corresponding angles states that if two lines are parallel, then any pair of corresponding angles created where a transversal crosses these lines are equal.

Given the problem: If one of the corresponding angles is a right angle, we need to explore if this necessitates that the other corresponding angle is also a right angle.

Let’s proceed with the steps to solve the problem:

  • Step 1: Recognize that we are discussing corresponding angles formed by a transversal cutting through two parallel lines.
  • Step 2: Apply the property that corresponding angles are equal when lines are parallel. This means if one angle in such a pair is a right angle, then the other must be equal to it.
  • Step 3: Since a right angle measures 9090^\circ, the other corresponding angle must also measure 9090^\circ since they are equal by the property of corresponding angles.

Therefore, based on the equality of corresponding angles when lines are parallel, if one corresponding angle is a right angle, the other angle will also be a right angle.

The final conclusion for the problem is that the statement is True.

Answer

True

Exercise #2

It is possible for two adjacent angles to be right angles.

Video Solution

Step-by-Step Solution

To determine if it is possible for two adjacent angles to be right angles, we start by considering the definition of adjacent angles. Adjacent angles share a common side and a common vertex. We must think about this scenario in terms of the angles lying on a straight line or a flat plane.

A right angle is exactly 9090^\circ. Hence, if we have two right angles that are adjacent, their measures would be:

  • First angle: 9090^\circ
  • Second angle: 9090^\circ

When these two angles are adjacent, as defined in the problem, their sum is:

90+90=180 90^\circ + 90^\circ = 180^\circ

Angles that are adjacent along a straight line add up exactly to 180180^\circ. Therefore, it is indeed possible for two adjacent angles to be both 9090^\circ. This configuration simply means that these two angles lie along a straight line, dividing it into two right angles.

Hence, the statement is True.

Answer

True

Exercise #3

The sum of adjacent angles is 180 degrees.

Video Solution

Step-by-Step Solution

To solve this problem, let's first understand the concept of adjacent angles. Adjacent angles share a common vertex and a common side. When two adjacent angles are formed by two intersecting lines, they often form what is known as a linear pair.

According to the Linear Pair Postulate, if two angles form a linear pair, then the sum of these adjacent angles is 180180 degrees. This is because these angles lie on a straight line, effectively forming a straight angle, which measures 180180 degrees.

Let's apply this knowledge to the statement in the problem:
The statement says, "The sum of adjacent angles is 180 degrees." In the context of the Linear Pair Postulate, this is indeed correct as adjacent angles that create a linear pair sum to 180180 degrees.

Therefore, when the statement refers specifically to linear pairs, it is true.

Thus, the solution to the problem is True.

Answer

True

Exercise #4

If one vertically opposite angle is acute, then the other will be obtuse.

Video Solution

Step-by-Step Solution

To solve this problem, we need to understand the properties of vertically opposite angles:

  • Vertically opposite angles are the angles that are opposite each other when two lines intersect.
  • One key property of vertically opposite angles is that they are always equal in measure.
  • An acute angle is defined as an angle that is less than 9090^\circ.
  • An obtuse angle is defined as an angle that is greater than 9090^\circ.

Given that vertically opposite angles are equal, if one angle is acute, the opposite angle must also be acute. This contradicts the statement in the problem that if one is acute, the other will be obtuse.

Therefore, the correct analysis of the problem reveals that the statement is incorrect.

Thus, the solution to the problem is False.

Answer

False

Exercise #5

Does the drawing show an adjacent angle?

Video Solution

Step-by-Step Solution

Adjacent angles are angles whose sum together is 180 degrees.

In the attached drawing, it is evident that there is no angle of 180 degrees, and no pair of angles can create such a situation.

Therefore, in the drawing there are no adjacent angles.

Answer

Not true

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