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4.3: Classify Triangles by Side Measurement

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    Classify triangles as scalene, isosoles or equalateral by identifying the number of equal sides.

    Triangle Classification by Side Lengths

    f-d_f7a42e31e1701b1190fdbb455734095e44d4609fb21430551db96d52+IMAGE_THUMB_POSTCARD_TINY+IMAGE_THUMB_POSTCARD_TINY.jpg
    Figure \(\PageIndex{1}\)

    Tasha goes sailing with her father on her father's old boat. The sail looks like a triangle. All of the sides of the sail are different lengths. Tasha wants to classify the triangle but she isn't sure how to name it. Given the side lengths of the triangle, how can Tasha classify the triangle?

    In this concept, you will learn how to classify triangles by their side lengths.

    Classifying Triangles by Side Lengths

    You can use the lengths of the sides to help you classify triangles.

    Let’s look at how to classify triangles according to side length.

    An equilateral triangle has side lengths that are the same. Here is an example.

    f-d_cc5fa1c45ad3f1aaf6ce335377f847c3f0ee62dade3cd5743d8e422a+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{2}\)

    These little lines let you know that the side lengths are the same. Sometimes you will see these and sometimes you won’t. You may have to figure it out on your own or by measuring with a ruler.

    A scalene triangle is a triangle where the lengths of all three sides are different. Here is an example of a scalene triangle.

    f-d_9e8aeb32612cdf5d5529878f2cd09b8f2779535f7d9fc2971c5e19ba+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{3}\)

    You can see that all three sides of the triangle are different lengths.

    An isosceles triangle has two side lengths that are the same and one side length that is different. Here is an example of an isosceles triangle.

    f-d_c453e355cf06319063c1ab21d4bd7a660ba7b0a49e515ad6a1bf867e+IMAGE_THUMB_POSTCARD_TINY+IMAGE_THUMB_POSTCARD_TINY.pngFigure \(\PageIndex{4}\)

    Example \(\PageIndex{1}\)

    Earlier, you were given a problem about Tasha and her sail.

    Her sail is shaped like a triangle with different side lengths. What classification should Tasha give the triangle?

    Solution

    First, determine if any of the side lengths are the same.

    No

    Then, classify the triangle.

    Scalene

    The answer is a scalene triangle.

    Example \(\PageIndex{2}\)

    Classify this triangle as scalene, isosceles or equilateral according to its side lengths.

    Side lengths, 6 cm, 4 cm, 6 cm

    Angles 70, 70, 40 degrees

    Solution

    First, determine if any of the side lengths are the same.

    Yes

    Next, determine how many side lengths are the same.

    Two

    Then, classify the triangle.

    Isosceles

    The answer is an isosceles triangle.

    Example \(\PageIndex{3}\)

    Classify this triangle according to its side lengths.

    f-d_df9d96f7e5714f757c190e1f930a052a20eff268d5e106c9b6f31df3+IMAGE_TINY+IMAGE_TINY.png
    Figure \(\PageIndex{4}\)

    Solution

    First, determine if any of the side lengths are the same.

    No,

    Then, classify the triangle.

    Scalene

    The answer is a scalene triangle.

    Example \(\PageIndex{4}\)

    Classify the triangle according to its side lengths.

    f-d_b7cf7550deb5de1a39539215512a02cc847d9b60cb2abaa83453b981+IMAGE_THUMB_POSTCARD_TINY+IMAGE_THUMB_POSTCARD_TINY.pngFigure \(\PageIndex{5}\)

    Solution

    First, determine if any of the side lengths are the same.

    Yes

    Next, determine how many side lengths are the same.

    All of the side lengths are the same

    Then, classify the triangle.

    Equilateral

    The answer is an equilateral triangle.

    Example \(\PageIndex{5}\)

    Classify the triangle according to its side lengths.

    f-d_f76aad14cee156b156c9e2f60a036a2d2c89bd9fd5e3e11f27c4d338+IMAGE_THUMB_POSTCARD_TINY+IMAGE_THUMB_POSTCARD_TINY.jpg
    Figure \(\PageIndex{6}\)

    Solution

    First, determine if any of the side lengths are the same.

    Yes

    Next, determine how many side lengths are the same.

    Two

    Then, classify the triangle.

    Isosceles

    The answer is an isosceles triangle.

    Review

    Answer the following questions using what you have learned about triangles their angles and side lengths.

    1. If a triangle is a right triangle, then how many angles are acute?
    2. How many angles in a right triangle are right angles?
    3. How many degrees are there in a right triangle?
    4. What is an obtuse angle?
    5. How many obtuse angles are in an obtuse triangle?
    6. If there is one obtuse angle, how many angles are acute?
    7. If a triangle is equiangular, what is the measure of all three angles?
    8. What does the word “interior angle” mean?
    9. True or false. The side lengths of a scalene triangle are all equal.
    10. True or false. The side lengths of a scalene triangle are all different.
    11. True or false. The side lengths of an equilateral triangle are all equal.
    12. True or false. An isosceles triangle has two side lengths the same and one different.
    13. True or false. A scalene triangle can also be an isosceles triangle.
    14. True or false. An equilateral triangle is also equiangular.
    15. True or false. A scalene triangle can not be an acute triangle.

    Review (Answers)

    To see the Review answers, open this PDF file and look for section 9.8.

    Vocabulary

    Term Definition
    Isosceles Triangle An isosceles triangle is a triangle in which exactly two sides are the same length.
    Scalene Triangle A scalene triangle is a triangle in which all three sides are different lengths.
    Triangle A triangle is a polygon with three sides and three angles.

    Additional Resources

    Interactive Element

    Video: Angle Relationships and Types of Triangles

    Practice: Classify Triangles by Side Measurement

    This page titled 4.3: Classify Triangles by Side Measurement is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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