Types of Triangles

Triangles are such a common shape in our everyday lives, but have you ever noticed how some triangles seem to look more similar than others? Well, ancient mathematicians noticed too, and over the years it has come to be accepted that there are four classifications of triangles.

Types of Triangles Types of Triangles

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    Triangle Classifications define triangles by their geometrical properties. These classifications are equilateral, isosceles, right-angled, and scalene.

    However, before we start classifying triangles, we must first discuss exactly what these properties are that define the classification of a given triangle.

    Types of Triangles Properties

    As discussed previously, triangles are classified by their geometric properties. The two geometric properties by which triangles are classified are the sides' lengths and interior angles.

    As can be seen in the diagram below, the interior angles of the triangle are the enclosed angles formed by each pair of the triangle's sides. These angles are denoted by α, β and γ(lowercase Greek letters).

    It is true of all triangles that their interior angles add up to 180°.

    Each side of the triangle has a length, denoted by a, b and c. The individual lengths of these sides do not determine the triangle's classification, in fact, they can have any length at all. It is in fact the length of the sides compared to each other that is important.

    Triangle classifications, triangle angles, triangle side lengths. StudySmarter

    A triangle highlighting its interior angles and side lengths, StudySmarter Originals

    With these two geometric properties, it is possible to put any triangle into one of our four classifications, and in many cases, we only need one or the other!

    Definitions of Types of Triangles

    As previously stated, the four types of triangles are equilateral, isosceles, scalene and right-angled. Each of these is a triangle you will have come across before, but maybe didn't know it! So let's see just what each of them is.

    Equilateral Triangles

    The most simple classification of a triangle is the equilateral triangle. The name here hints at how this type of triangle is defined. Equi is a common prefix arising from the adjective equal, and so words beginning with this prefix often describe things that are equal. For instance, if two shops are equidistant from where you are, it means they are each the same distance away. Equilateral triangles are no different!

    An equilateral triangle is a triangle with three sides of equal length. Consequently, the interior angles of an equilateral triangle are also equal to one another.

    The triangle below is an equilateral triangle, immediately we can tell this by the single dashes on each side which signify that the sides are of equal length, i.e. a = b = c. The interior angles of the triangle can also be seen to be equal, with each being60°, but is this always the case?

    Triangle classifications, triangle angles, triangle side lengths. equilateral triangle, types of triangle StudySmarterAn equilateral triangle, characterised by its three equal sides and three equal interior angles, StudySmarter Originals

    As we know, a triangle's interior angles will always all add up to 180°. Let's say that each angle in the equilateral triangle is α.

    180° = 3 × α

    Dividing both sides by three we can find the value of α.

    α = 60°

    So there we have it, the interior angles of an equilateral triangle are always each 60°!

    Isosceles Triangles

    The next type of triangle we will look at is the isosceles triangle. Similar to equilateral triangles, we can spot an isosceles triangle by the length of its sides or by the size of its interior angles.

    An isosceles triangle is a triangle with two sides of equal length, and a third of a different length. Consequently, only two of the interior angles of an isosceles triangle are equal.

    The triangle below is an isosceles triangle. In this case, the dashes indicate that only the sides a and b are equal. The interior angles β and γ can be seen to be equal, but not the angle α.

    Triangle classifications, triangle angles, triangle side length, isosceles triangle, types of triangle StudySmarterAn isosceles triangle, characterised by its two equal sides and two equal angles, StudySmarter Originals

    Scalene Triangles

    So, we've seen equilateral triangles, which have three equal sides, and isosceles triangles which have two equal sides, so what will be the case for scalene triangles? You guessed it, they have no equal sides!

    A scalene triangle is a triangle with no sides of equal length. Consequently, none of the interior angles are equal.

    The triangle below is a scalene triangle. As such, it has no dashes indicating equal sides.

    Triangle classifications, triangle angles, triangle side length, scalene triangle, types of triangle StudySmarterA scalene triangle, characterised by its lack of equal sides or interior angles, StudySmarter Originals

    Right-Angled Triangles

    The final type of triangle is a right-angled triangle. Unlike the previous types of triangles discussed, right-angled triangles are not defined by the number of equal sides or the number of equal angles. In fact, the only thing that a triangle needs to possess to be right-angled, is one interior angle equalling 90°. In other words, it must possess a right angle. This means that a right-angled triangle will also be either an isosceles triangle or a scalene triangle.

    A right-angled triangle is a triangle possessing one interior angle of 90°. Its other two interior angles may be equal or not equal, and it may have two or zero equal sides.

    The triangle below is a right-angled triangle. This can be instantly recognised by the box in place of the usual angle segment, denoting a 90° angle.

    Triangle classifications, triangle angles, triangle side length, right-angled triangle, types of triangle StudySmarterA right-angled triangle, characterised by its single right interior angle, StudySmarter Originals

    Right-angled triangles are extremely important in maths, why not head over to our explanations on Pythagoras Theorem and Trigonometry to really learn about what makes them special!

    Types of Triangles Examples

    Now, let's see if we can use what we've learnt to try and classify some triangles.

    What triangle classification does each of the triangles below belong to?

    a)

    Finding the type of a triangle, classifications of triangles, isosceles triangle studysmarterTriangle for question a) with three labelled angles, StudySmarter Originals

    Solution:

    This triangle is an isosceles triangle, as it has two angles that are equal, β and γ, and a third that is not.

    b)

    Finding the type of a triangle, classifications of triangles, equilateral triangle studysmarter

    Triangle for question b) with three labelled sides, StudySmarter Originals

    Solution:

    This triangle is an equilateral triangle, as it has three sides of equal length.

    c)

    Finding the type of a triangle, classifications of triangles, right angled triangle studysmarterTriangle for question c), with two labelled angles, StudySmarter Originals

    Solution:

    We are only given two angles for this triangle; however, knowing that the interior angles of a triangle always add up to 180°, we can determine the third angle, γ. Firstly, we equate the sum of the three angles to 180°.

    180° = α + β + γ

    Then, we substitute the angles we know and rearrange the equation to find γ.

    180° = 64° + 26° + γ

    γ = 180° - 64° - 26°

    γ = 90°

    As the third angle, γ, is a right angle, the triangle is a right-angled triangle. As all three interior angles are different, it is also a scalene triangle.

    d)

    Finding the type of a triangle, classifications of triangles, scalene triangle studysmarterTriangle for question d), with three labelled angles, StudySmarter

    Solution:

    This triangle is a scalene triangle as none of its interior angles are equal.

    e)

    Finding the type of a triangle, classifications of triangles, isosceles triangle studysmarter

    Triangle for question e) with no labelled angles or sides, StudySmarter Originals

    Solution:

    The triangle is an isosceles triangle, as it has two sides of the same length, indicated by the two dashes.

    f)

    Finding the type of a triangle, classifications of triangles, right-angled triangle studysmarterTriangle for question f), with two labelled angles, StudySmarter

    Solution:

    In this question, we are given two interior angles which are equal. By remembering that all interior angles in a triangle add up to 180°, we can use these two interior angles to find the third angle, γ.

    First, we equate the three interior angles to 180°.

    180° = α + β + γ

    Then, we substitute the angles we know and rearrange the equation to find γ.

    180° = 45° + 45° + γ

    γ = 180° - 45° - 45°

    γ = 90°

    As the triangle has one right interior angle, and two equal interior angles, it must be both an isosceles, and a right-angled triangle.

    Types of Triangles - Key takeaways

    • Triangles can be classified by comparing the length of their sides, and by comparing the size of their interior angles.
    • There are four types of triangles: equilateral, isosceles, scalene, and right-angled.
    • Equilateral triangles have three equal sides and three equal interior angles.
    • Isosceles triangles have two equal sides and two equal interior angles.
    • Scalene triangles have no equal sides and no equal interior angles.
    • Right-angled triangles have one interior angle of 90°.
    Types of Triangles Types of Triangles
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    Frequently Asked Questions about Types of Triangles

    What are the types of triangles in geometry?

    There are four types of triangles: equilateral, isosceles, scalene, and right-angled.

    What are the different types of triangles and their properties?

    Equilateral triangles have three equal sides and three equal interior angles.

    Isosceles triangles have two equal sides and two equal interior angles.

    Scalene triangles have no equal sides and no equal interior angles.

    Right-angled triangles have one interior angle which is a right-angle.

    How to classify the types of triangles?

    Triangles can be classified by comparing the lengths of their sides, as well as by comparing the size of their interior angles.

    What are the 3 types of triangles based on sides?

    Equilateral, isosceles, and scalene triangles are all classified based on the length of their sides.

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