Alternate angles

Definition:

Alternate angles are on opposite sides of the transversal that intersects two parallel lines and are not on the same side of the parallel lines to which they belong.
Alternate angles are equal.

The following sketch illustrates two pairs of alternate angles, one is painted red and the other blue.

two pairs of alternate angles, one is painted red and the other blue.

Identifying Alternate Angles:

There are two types of alternate angles: alternate interior (inside the parallel lines) and alternate exterior (outside the parallel lines). These angles are always equal when the lines are parallel.

The Significance of Parallel Lines:

For alternate angles to be congruent, the lines must be parallel. Recognizing this can help in solving various geometric problems and proofs, as it provides essential information about the relationships between lines and angles.

Other Angles:

In addition to alternate angles, several other angle relationships occur when a transversal crosses parallel lines.

Suggested Topics to Practice in Advance

  1. Parallel lines

Practice Alternate angles

Examples with solutions for Alternate angles

Exercise #1

Identify the angle shown in the figure below?

Step-by-Step Solution

Remember that adjacent angles are angles that are formed when two lines intersect each other.

These angles are created at the point of intersection, one adjacent to the other, and that's where their name comes from.

Adjacent angles always complement each other to one hundred and eighty degrees, meaning their sum is 180 degrees. 

Answer

Adjacent

Exercise #2

In which of the diagrams are the angles α,β  \alpha,\beta\text{ } vertically opposite?

Step-by-Step Solution

Remember the definition of angles opposite by the vertex:

Angles opposite by the vertex are angles whose formation is possible when two lines cross, and they are formed at the point of intersection, one facing the other. The acute angles are equal in size.

The drawing in answer A corresponds to this definition.

Answer

αααβββ

Exercise #3

Is it possible to have two adjacent angles, one of which is obtuse and the other right?

Video Solution

Step-by-Step Solution

Remember the definition of adjacent angles:

Adjacent angles always complement each other up to one hundred eighty degrees, that is, their sum is 180 degrees.

This situation is impossible since a right angle equals 90 degrees, an obtuse angle is greater than 90 degrees.

Therefore, together their sum will be greater than 180 degrees.

Answer

No

Exercise #4

a a is parallel to

b b

Determine which of the statements is correct.

αααβββγγγδδδaaabbb

Video Solution

Step-by-Step Solution

Let's review the definition of adjacent angles:

Adjacent angles are angles formed where there are two straight lines that intersect. These angles are formed at the point where the intersection occurs, one next to the other, and hence their name.

Now let's review the definition of collateral angles:

Two angles formed when two or more parallel lines are intersected by a third line. The collateral angles are on the same side of the intersecting line and even are at different heights in relation to the parallel line to which they are adjacent.

Therefore, answer C is correct for this definition.

Answer

β,γ \beta,\gamma Colateralesγ,δ \gamma,\delta Adjacent

Exercise #5

What angles are shown in the diagram below?

Step-by-Step Solution

Let's remember that vertical angles are angles that are formed when two lines intersect. They are are created at the point of intersection and are opposite each other.

Answer

Vertical

Exercise #6

Which type of angles are shown in the diagram?

Step-by-Step Solution

First let's remember that corresponding angles can be defined as a pair of angles that can be found on the same side of a transversal line that intersects two parallel lines.

Additionally, these angles are positioned at the same level relative to the parallel line to which they belong.

Answer

Corresponding

Exercise #7

Which type of angles are shown in the figure below?

Step-by-Step Solution

Alternate angles are a pair of angles that can be found on the opposite side of a line that cuts two parallel lines.

Furthermore, these angles are located on the opposite level of the corresponding line that they belong to.

Answer

Alternate

Exercise #8

Identify the angles in the image below:

Step-by-Step Solution

Given that the three lines are parallel to one another, one should note that corresponding angles can be defined as a pair of angles that can be found on the same side of a line intended to intersect two parallel lines.

Additionally, these angles are located on the same level relative to the line they correspond to.

Answer

Corresponding

Exercise #9

Identify the angles marked in the figure below given that ABCD is a trapezoid:

AAABBBCCCDDDEEEFFF

Step-by-Step Solution

Since we know that ABCD is a trapezoid, lines AB and CD are parallel to each other.

Let's remember that alternate angles are defined as a pair of angles that can be found in the opposite aspect of a line intended to intersect two parallel lines.

Additionally, these angles are positioned at opposite levels relative to the parallel line to which they belong.

Answer

Alternates

Exercise #10

Look at the rhombus in the figure.

What is the relationship between the marked angles?

BAAB

Step-by-Step Solution

Let's remember the different definitions of angles:

Corresponding angles are angles located on the same side of the line that intersects the two parallels and are also situated at the same level with respect to the parallel line they are adjacent to.

Therefore, according to this definition, these are the angles marked with the letter A

Alternate angles are angles located on two different sides of the line that intersects two parallels, and which are also not at the same level with respect to the parallel they are adjacent to.

Therefore, according to this definition, these are the angles marked with the letter B

Answer

A - corresponding; B - alternate

Exercise #11


Look at the rectangle ABCD below.

What type of angles are labeled with the letter A in the diagram?

What type are marked labeled B?

AAABBBCCCDDDBBAA

Step-by-Step Solution

Let's remember the definition of corresponding angles:

Corresponding angles are angles located on the same side of the line that cuts through the two parallels and are also situated at the same level with respect to the parallel line to which they are adjacent.

It seems that according to this definition these are the angles marked with the letter A.

Let's remember the definition of adjacent angles:

Adjacent angles are angles whose formation is possible in a situation where there are two lines that cross each other.

These angles are formed at the point where the intersection occurs, one next to the other, and hence their name.

Adjacent angles always complement each other to one hundred eighty degrees, that is, their sum is 180 degrees.

It seems that according to this definition these are the angles marked with the letter B.

Answer

A - corresponding

B - adjacent

Exercise #12

The lines a and b are parallel.

What are the corresponding angles?

αααβββγγγδδδaaabbb

Video Solution

Step-by-Step Solution

Given that line a is parallel to line b, let us remind ourselves of the definition of corresponding angles between parallel lines:

Corresponding angles are angles located on the same side of the line that intersects the two parallels and are also situated at the same level with respect to the parallel line to which they are adjacent.

Corresponding angles are equal in size.

According to this definition α=β \alpha=\beta and as such they are the corresponding angles.

Answer

α,β \alpha,\beta

Exercise #13

What angles are described in the drawing?

Step-by-Step Solution

Since the angles are not on parallel lines, none of the answers are correct.

Answer

Ninguna de las respuestas

Exercise #14

What are alternate angles of the given parallelogram ?

αααγγγδδδβββxxx

Step-by-Step Solution

To solve the question, we must first remember that a parallelogram has two pairs of opposite sides that are parallel and equal.

That is, the top line is parallel to the bottom one.

From this, it is easy to identify that angle X is actually an alternate angle of angle δ, since both are on different sides of parallel straight lines.

Answer

δ,χ \delta,\chi

Exercise #15

What type of angles are shown in the diagram below?

Step-by-Step Solution

Since we are not given any data related to the lines, we cannot determine whether they are parallel or not.

As a result, none of the options are correct.

Answer

None of the above