If we add a third line that intersects the two parallel lines (those lines that could never cross), we will obtain various types of angles. To classify these angles we must observe if they are: above the line - the pink part below the line - the light blue part to the right of the line - the red part to the left of the line - the green part
The corresponding angles located between parallel lines are equal. They are called corresponding angles because:
they are on the same side of the transversal
they are on the same "floor" in relation to the line
Here are some examples of corresponding angles:
The two angles marked are on the left side and on the ground floor - they are corresponding and equivalent.
Vertically Opposite Angles
The vertically opposite angles that are located between parallel lines are equal. They are called vertically opposite angles because:
They share the same vertex - they are located on the same vertex
They are opposite each other
Here are some examples of vertically opposite angles:
The two marked angles are located on the same vertex and are opposite each other, therefore, they are vertically opposite angles.
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Test your knowledge
Question 1
Does the diagram show an adjacent angle?
Incorrect
Correct Answer:
Yes
Question 2
Does the diagram show an adjacent angle?
Incorrect
Correct Answer:
Yes
Question 3
Does the diagram show an adjacent angle?
Incorrect
Correct Answer:
No
Adjacent Angles
The sum of adjacent angles located between parallel lines is equal to 180. They are called adjacent angles because:
They are next to each other
They are on the same line
Here are some examples of adjacent angles:
The two marked angles are on the same line - (diagonal) - and are next to each other, therefore, they are adjacent angles.
Alternate Angles
The alternate angles that are located between parallel lines are equal. They are called alternate angles because:
they are not on the same side of the transversal
they are not on the same "level" in relation to the line
Here are some examples of alternate angles:
The two marked angles are on different "levels" and sides, therefore, they are alternate angles.
Do you know what the answer is?
Question 1
Does the diagram show an adjacent angle?
Incorrect
Correct Answer:
No
Question 2
Does the diagram show an adjacent angle?
Incorrect
Correct Answer:
No
Question 3
If two adjacent angles are not right angles, then one of them is obtuse and the other is acute.
Incorrect
Correct Answer:
True
Collateral angles
The sum of consecutive angles located between parallel lines is equal to 180. They are called consecutive angles because:
they are on the same side of the transversal
but they are not on the same "level" in relation to the line
Here are some examples of consecutive angles:
The two marked angles are on the same side of the line, but at a different height, therefore, they are consecutive angles. Observe: The angles painted in red in the illustration above are external consecutive angles since they are on the outer side of the parallel lines The internal consecutive angles are on the inner side of the parallel lines:
Angles on Parallel Lines
Now to practice!
Give an example according to the illustration of:
Alternate angles
Corresponding angles
Vertically opposite angles
Adjacent angles
Consecutive interior angles
Consecutive exterior angles
Solution:
Examples of alternate angles 1,8 Both are on different sides and levels, therefore, they are alternate.
Examples of corresponding angles 8,4 Both are on the same side and at the same level or floor, therefore, they are corresponding.
Examples of vertically opposite angles 1,4 Both share the same vertex and are located opposite each other, therefore, they are vertically opposite angles.
Examples of adjacent angles 7,8 Both are on the same line and are located next to each other, therefore, they are adjacent.
Examples of exterior alternate angles 1,7 Both are on the same side, but not at the same level. In addition, they are located on the outside of the line, therefore, they are exterior alternate angles.
Examples of interior alternate angles 3,5 Both are on the same side, but not at the same level. In addition, they are located on the inside of the line, therefore, they are interior alternate angles.
Another exercise:
What are the marked angles called in the illustration?
Solution
The marked angles are alternate They are located on different sides and heights, therefore, they are alternate.
Another exercise:
What are the angles shown in the illustration called?
Solution
The indicated angles are consecutive They are on the same side of the line, but at different heights, therefore, they are external consecutive angles.
Another exercise:
What are the angles shown in the illustration called?
Solution
The indicated angles are adjacent They are on the same blue line and are next to each other, therefore, they are adjacent angles.
Examples and exercises with solutions of right angles and parallels
Exercise #1
Does the diagram show an adjacent angle?
Video Solution
Step-by-Step Solution
To determine if the diagram shows adjacent angles, we need to analyze the geometric arrangement shown:
Step 1: Identify the common vertex.
In the diagram, both the vertical line and the diagonal line intersect at a point. This intersection point serves as the common vertex for the angles in question, as they radiate outward from this shared point.
Step 2: Identify the common side.
Adjacent angles must share a common side or arm. In the diagram, the vertical line acts as one common side for both angles, with one angle extending upwards and the other horizontally from the vertex.
Step 3: Ensure no overlap of interiors.
It is equally essential to ensure that these two angles do not overlap. Each angle branches from the vertex in a different direction, maintaining distinct interiors.
By confirming the presence of a common vertex and a common side without overlap of the angle interiors, the angles satisfy the definition of being adjacent.
Therefore, the diagram does indeed show adjacent angles.
Consequently, the correct answer is Yes.
Answer
Yes
Exercise #2
Does the diagram show an adjacent angle?
Video Solution
Step-by-Step Solution
To determine whether the diagram shows adjacent angles, we need to confirm the presence of two properties:
1. Two angles must share a common vertex.
2. These angles must have a common arm and should not overlap.
Based on the given representation, the provided diagram consists solely of a single line. There are no visible intersecting lines or vertices from which angles can originate. Without intersection, there cannot be distinct angles, and thereby no adjacent angles can be identified.
Therefore, the diagram lacks the necessary properties to demonstrate adjacent angles. Hence, the correct choice is No.
Answer
No
Exercise #3
Does the diagram show an adjacent angle?
Video Solution
Step-by-Step Solution
To solve this problem, we'll follow these steps:
Step 1: Inspect the given diagram for angles.
Step 2: Determine if any angles share a common vertex and a common side.
Step 3: Verify that the angles do not overlap.
Now, let's work through each step:
Step 1: Inspecting the diagram, we notice several intersecting lines.
Step 2: To check for adjacent angles, we look for pairs of angles that share both a common vertex and a common side. An adjacent angle must be formed by such pairs, ensuring they do not overlap.
Step 3: Based on our definition, after closely examining the diagram, no pair of angles in the diagram seems to satisfy the definition of adjacent angles. The intersecting lines form angles that don't share a common arm with any other angle at the same vertex in the manner required for adjacency.
Therefore, the solution to the problem is No, the diagram does not show an adjacent angle.
Answer
No
Exercise #4
If two adjacent angles are not right angles, then one of them is obtuse and the other is acute.
Video Solution
Step-by-Step Solution
To solve the problem, let’s consider the nature of adjacent angles:
Step 1: Adjacent angles are two angles that share a common side and vertex. If two adjacent angles form a straight line, their measures sum up to 180∘.
Step 2: According to the problem, neither angle is a right angle, meaning neither is 90∘.
Step 3: Given this constraint, analyze the possibilities:
If one angle is acute (less than 90∘), then the other must be more than 90∘ to make the total 180∘. Therefore, the other angle is obtuse.
If one angle is obtuse (greater than 90∘), then the other must be less than 90∘ to make the total 180∘. Thus, the other angle is acute.
Since both scenarios involve one angle being acute and the other obtuse, we verify that the statement is correct.
Therefore, the statement is true.
Answer
True
Exercise #5
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 90∘. Hence, if we have two right angles that are adjacent, their measures would be:
First angle: 90∘
Second angle: 90∘
When these two angles are adjacent, as defined in the problem, their sum is:
90∘+90∘=180∘
Angles that are adjacent along a straight line add up exactly to 180∘. Therefore, it is indeed possible for two adjacent angles to be both 90∘. 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
Check your understanding
Question 1
It is possible for two adjacent angles to be right angles.
Incorrect
Correct Answer:
True
Question 2
It is possible for two adjacent angles to be obtuse.
Incorrect
Correct Answer:
False
Question 3
Is it possible to have two adjacent angles, one of which is obtuse and the other right?