Congruent triangles - Examples, Exercises and Solutions

Congruent triangles are identical triangles.

That means in triangles whose angles and sides are equal, their area and perimeter will also be equal.

But keep in mind that this case is different from when triangles are similar, that is, when the angles are equal but the side lengths are different in the corresponding ratio.

Congruence Criteria

To prove that 2 triangles are congruent we can use one of the following criteria:

  • SAS - side, angle, side
  • ASA - angle, side, angle
  • SSS- side, side, side
  • SSA- side, side, angle

By verifying one of the triangle congruence criteria, we can affirm that the triangles are congruent.

Practice Congruent triangles

Exercise #1

Look at the triangles in the diagram.

Which of the following statements is true?

535353535353101010131313131313101010AAABBBCCCDDDEEEFFF

Step-by-Step Solution

According to the existing data:

EF=BA=10 EF=BA=10 (Side)

ED=AC=13 ED=AC=13 (Side)

The angles equal to 53 degrees are both opposite the greater side (which is equal to 13) in both triangles.

(Angle)

Since the sides and angles are equal among congruent triangles, it can be determined that angle DEF is equal to angle BAC

Answer

Angles BAC is equal to angle DEF.

Exercise #2

Given: ΔABC isosceles

and the line AD cuts the side BC.

Are ΔADC and ΔADB congruent?

And if so, according to which congruence theorem?

AAABBBCCCDDD

Step-by-Step Solution

As we know the triangle is isosceles, then AC=AB

AD=AD since it is a common side to the triangles ADC and ADB

Given that the line AD intersects side BC, and therefore BD=DC

Therefore, the triangles are congruent according to the SSS (side, side, side) theorem

Answer

Congruent by L.L.L.

Exercise #3

Choose the pair of triangles that are congruent according to S.S.S.

Step-by-Step Solution

In answer A, we are given two triangles with different angles, therefore the sides are also different and they are not congruent according to S.S.S.

In answer B, we are given two right triangles, but their angles are different and so are the sides. Therefore, they are not congruent according to S.S.S.

In answer D, we do not have enough data, therefore it is not possible to determine that they are congruent according to S.S.S.

In answer C, we see that all the sides are equal to each other in both triangles and therefore they are congruent according to S.S.S.

Answer

879879

Exercise #4

Look at the triangles in the diagram.

Determine which of the statements is correct.

343434343434555444444555AAABBBCCCDDDEEEFFF

Step-by-Step Solution

Let's consider that:

AC=EF=4

DF=AB=5

Since 5 is greater than 4 and the angle equal to 34 is opposite the larger side in both triangles, then the angle ACB is equal to the angle DEF

Therefore, the triangles are congruent according to the SAS theorem, as a result of this all angles and sides are congruent, and all answers are correct.

Answer

All of the above.

Exercise #5

The triangles ABO and CBO are congruent.

Which side is equal to BC?

AAABBBCCCDDDOOO

Video Solution

Step-by-Step Solution

Let's consider the corresponding congruent triangles letters:

CBO=ABO CBO=ABO

That is, from this we can determine:

CB=AB CB=AB

BO=BO BO=BO

CO=AO CO=AO

Answer

Side AB

Exercise #1

Look at the triangles in the diagram.

Which of the following statements is true?

242424242424444666666444AAACCCBBBEEEFFFDDD

Step-by-Step Solution

This question actually has two steps:

In the first step, you must define if the triangles are congruent or not,

and then identify the correct answer among the options.

 

Let's look at the triangles: we have two equal sides and one angle,

But this is not the angle between them, therefore, it cannot be proven according to the S.A.S theorem

Remember the fourth congruence theorem - S.A.A
If the two triangles are equal to each other in terms of the lengths of the two sides and the angle opposite to the side that is the largest, then the triangles are congruent.

 

But the angle we have is not opposite to the larger side, but to the smaller side,

Therefore, it is not possible to prove that the triangles are congruent and no theorem can be established.

Answer

It is not possible to calculate.

Exercise #2

Triangles ABC and CDA are congruent.

Which angle is equal to angle BAC?

AAABBBEEECCCDDD

Video Solution

Step-by-Step Solution

We observe the order of the letters in the congruent triangles and write the matches (from left to right).

ABC=CDA ABC=CDA

That is:

Angle A is equal to angle C.

Angle B is equal to angle D.

Angle C is equal to angle A.

From this, it is deduced that angle BAC (where the letter A is in the middle) is equal to angle C — that is, to angle DCA (where the letter C is in the middle).

Answer

C

Exercise #3

Are the triangles in the image congruent?

If so, according to which theorem?

393939393939555777777555

Step-by-Step Solution

Although the lengths of the sides are equal in both triangles, we observe that in the right triangle the angle is adjacent to the side whose length is 7, while in the triangle on the left side the angle is adjacent to the side whose length is 5.

Since it's not the same angle, the angles between the triangles do not match and therefore the triangles are not congruent.

Answer

No.

Exercise #4

What data must be added so that the triangles are congruent?

656565555888555888AAABBBCCCDDDEEEFFF

Step-by-Step Solution

Let's consider that:

DF = AC = 8

DE = AB = 5

8 is greater than 5, therefore the angle DEF is opposite the larger side and is equal to 65 degrees.

That is, the figure we are missing is the angle of the second triangle.

We will examine which angle is opposite the large side AC.

ABC is the angle opposite the larger side AC so it must be equal to 65 degrees.

Answer

Angle ABC equals 65.

Exercise #5

The triangles ABO and CBO are congruent.

Which side is equal to BC?

AAABBBCCCDDDOOO

Video Solution

Step-by-Step Solution

Let's consider the corresponding congruent triangles letters:

CBO=ABO CBO=ABO

That is, from this we can determine:

CB=AB CB=AB

BO=BO BO=BO

CO=AO CO=AO

Answer

Side AB

Exercise #1

Which of the triangles are congruent?

454545454545454545IIIIII

Step-by-Step Solution

Let's observe the angle in each of the triangles and note that each time it is opposite to the length of a different side.

Therefore, none of the triangles are congruent since it is impossible to know from the data.

Answer

It is not possible to know based on the data.

Exercise #2

Are similar triangles necessarily congruent?

Video Solution

Step-by-Step Solution

There are similar triangles that are not necessarily congruent, so this statement is not correct.

Answer

No

Exercise #3

Are the triangles shown in the diagram congruent? If so, according to which congruence theorem?

120°120°120°120°120°120°161616999161616999AAABBBCCCGGGFFFDDD

Step-by-Step Solution

To answer the question, we need to know the fourth congruence theorem: S.A.S.

The theorem states that triangles are congruent when they have an equal pair of sides and an equal angle.

However, there is one condition: the angle must be opposite the longer side of the triangle.

 

We start with the sides:

DF = CB = 16
GD = AC = 9

Now, we look at the angles:

A = G = 120

We know that an angle of 120 is an obtuse angle and this type of angle is always opposite the larger side of the triangle.

Therefore, we can argue that the triangles are congruent according to the S.A.S. theorem.

 

Answer

Congruent according to S.A.S.

Exercise #4

Triangles ABC and CDA are congruent.

Which angle is equal to angle BAC?

AAABBBEEECCCDDD

Video Solution

Step-by-Step Solution

We observe the order of the letters in the congruent triangles and write the matches (from left to right).

ABC=CDA ABC=CDA

That is:

Angle A is equal to angle C.

Angle B is equal to angle D.

Angle C is equal to angle A.

From this, it is deduced that angle BAC (where the letter A is in the middle) is equal to angle C — that is, to angle DCA (where the letter C is in the middle).

Answer

C

Exercise #5

Are the triangles in the drawing congruent?

303030303030X+2X+2X+23333332X+4

Step-by-Step Solution

For triangles to be congruent, it is necessary to demonstrate that the S.A.S theorem is satisfied

We have a common side whose length in both triangles is equal to 3.

Now we will look for the lengths of the other sides:

2X+4=X+2 2X+4=X+2

We proceed with the sections accordingly:24=2XX 2-4=2X-X

2=X -2=X

We place it in the right triangle and will find the length of the side:2+2=0 -2+2=0

Since it is not possible for the length of a side to be equal to 0, the triangles are not congruent.

Answer

No