Isosceles triangle

🏆Practice types of triangles

Definition of isosceles triangle

The isosceles triangle is a type of triangle that has two sides (legs) of equal length.

A consequence of having two sides of equal length implies that also the two angles opposite these sides measure the same.

Key Parts:

  • Legs: The two equal sides
  • Base: The third side (different length)
  • Vertex angle: The angle between the two legs
  • Base angles: The two equal angles adjacent to the base

This fundamental property—that equal sides create equal opposite angles—makes isosceles triangles essential building blocks in geometry and forms the basis for the Isosceles Triangle Theorem.

A - Identification of an isosceles triangle

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Test yourself on types of triangles!

In a right triangle, the side opposite the right angle is called....?

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In an isosceles triangle the two equal sides are called legs and the third is called the base and in most exercises acts as the base.

The angle that lies between the two equal sides is called the angle at the vertex (or apex angle).

The two angles adjacent to the base are called base angles and measure the same.

Furthermore, the base angles that measure the same cannot be obtuse (more than 90° 90° ) or right angles (equal to 90° 90° ), because their measures would add up to at least 180°, therefore, they have to be acute (less than 90° 90° ).

The above causes the isosceles triangle to be further classified as obtuse, right or acute, depending on how the vertex angle is.

The Isosceles Triangle Theorem: In any isosceles triangle, the base angles are equal. Conversely, if two angles in a triangle are equal, then the triangle is isosceles.

Next, we will see some examples of isosceles triangles:

Isosceles triangle

A1 isosceles triangle

Examples of isosceles triangles

Examples of isosceles triangles

We will demonstrate the characteristics of isosceles triangles by means of an exercise.

Given the isosceles triangle KLM \triangle KLM as shown in the figure.


Example Question

Use the data shown in the illustration to calculate the angles L and M.

A6 - Isosceles triangle

We will start with the triangle KMS \triangle KMS . We already know two angles, so we can calculate the third angle M M (The sum of the interior angles of a triangle is 180° 180° degrees). Thus, the angle M M measures 50° 50° degrees (since 50°=180°100°30° 50° = 180° - 100° - 30° ).

Since we know that the triangle KLM \triangle KLM is isosceles we understand that its base angles L L and M M are equal.

Therefore L=M=50° L=M=50°


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Questions on the subject

What is an isosceles triangle?

It is a triangle that has two sides of equal length.


What is an acute isosceles triangle?

It is an isosceles triangle whose angle at the vertex measures less than90° 90° .


What is a right isosceles triangle?

It is an isosceles triangle whose angle at the vertex measures exactly 90° 90° .


If you are interested in learning more about other triangle topics, you can enter one of the following articles:

On the Tutorela blog you will find a variety of articles about mathematics.


Ejemplos y ejercicios con soluciones de triángulo isósceles

Exercise #1

In a right triangle, the side opposite the right angle is called....?

Step-by-Step Solution

The problem requires us to identify the side of a right triangle that is opposite to its right angle.
In right triangles, one of the most crucial elements to recognize is the presence of a right angle (90 degrees).
The side that is directly across or opposite the right angle is known as the hypotenuse. It is also the longest side of a right triangle.
Therefore, when asked for the side opposite the right angle in a right triangle, the correct term is the hypotenuse.

Selection from the given choices corroborates our analysis:

  • Choice 1: Leg - In the context of right triangles, the "legs" are the two sides that form the right angle, not the side opposite to it.
  • Choice 2: Hypotenuse - This is the correct identification for the side opposite the right angle.

Therefore, the correct answer is Hypotenuse \text{Hypotenuse} .

Answer

Hypotenuse

Exercise #2

In an isosceles triangle, what are each of the two equal sides called ?

Step-by-Step Solution

In an isosceles triangle, there are three sides: two sides of equal length and one distinct side. Our task is to identify what the equal sides are called.

To address this, let's review the basic properties of an isosceles triangle:

  • An isosceles triangle is defined as a triangle with at least two sides of equal length.
  • The side that is different in length from the other two is usually called the "base" of the triangle.
  • The two equal sides of an isosceles triangle are referred to as the "legs."

Therefore, each of the two equal sides in an isosceles triangle is called a "leg."

In our problem, we confirm that the correct terminology for these two equal sides is indeed "legs," distinguishing them from the "base," which is the unequal side. This aligns with both the typical definitions and properties of an isosceles triangle.

Thus, the equal sides in an isosceles triangle are known as legs.

Answer

Legs

Exercise #3

In a right triangle, the two sides that form a right angle are called...?

Step-by-Step Solution

In a right triangle, there are specific terms for the sides. The two sides that form the right angle are referred to as the legs of the triangle. To differentiate, the side opposite the right angle is called the hypotenuse, which is distinct due to being the longest side. Hence, in response to the problem, the sides forming the right angle are correctly identified as Legs.

Answer

Legs

Exercise #4

Does the diagram show an obtuse triangle?

Video Solution

Step-by-Step Solution

To determine if the triangle in the diagram is obtuse, we will visually assess the angles:

  • Step 1: Identify the angles in the diagram. The triangle has three angles, with one angle appearing between the horizontal base and the left slanted side.
  • Step 2: Evaluate the angle between the base and the left side. If it opens wider than a right angle, it's considered obtuse. This angle seems to be greater than 9090^\circ, indicating obtuseness.
  • Step 3: Conclude based on visual inspection. Since this key angle is greater than 9090^\circ, the triangle must be an obtuse triangle.

Therefore, the solution to the problem is Yes; the diagram does show an obtuse triangle.

Answer

Yes

Exercise #5

Does the diagram show an obtuse triangle?

Video Solution

Step-by-Step Solution

To determine if the triangle shown in the diagram is obtuse, we proceed as follows:

  • Step 1: Identify that the diagram is indeed a triangle by observing the confluence of three edges forming a closed shape.
  • Step 2: Appreciate the geometric arrangement of the triangle, focusing on the sides' lengths and angles visually.
  • Step 3: Noticeably, the longest side of the triangle represents a noticeable tilt indicating the presence of an obtuse angle.

Based on the observation above, notably from the triangle's longest side against the base, it's clear that one angle is larger than 9090^\circ. Hence, the triangle in the diagram is indeed an obtuse triangle.

Therefore, the correct answer is Yes.

Answer

Yes

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