http://www.ck12.org Chapter 1. Basics of Geometry
Isosceles Triangle:Whenat leasttwo sides of a triangle are congruent.
Equilateral Triangle:When all sides of a triangle are congruent.
Note that by the above definitions, an equilateral triangle is also an isosceles triangle.
Example A
Which of the figures below are not triangles?
Bis not a triangle because it hasone curved side.Dis not a closed shape, so it is not a triangle either.
Example B
Which term best describes 4 RSTbelow?
This triangle has one labeled obtuse angle of 92◦. Triangles can only have one obtuse angle, so it is an obtuse
triangle.
Example C
Classify the triangle by its sides and angles.
We are told there are two congruent sides, so it is an isosceles triangle. By its angles, they all look acute, so it is an
acute triangle. Typically, we say this is an acute isosceles triangle.
Watch this video for help with the Examples above.
MEDIA
Click image to the left for more content.CK-12 Foundation: Chapter1TriangleClassificationB
Vocabulary
Atriangleis any closed figure made by three line segments intersecting at their endpoints. Every triangle has three
vertices(the points where the segments meet), threesides(the segments), and threeinterior angles(formed at each
vertex). Aright triangleis a triangle with one right angle. Anobtuse triangleis a triangle with one obtuse angle.
Anacute triangleis a triangle where all three angles are acute. Anequiangular triangleis a triangle with all
congruent angles. Ascalene triangleis a triangle where all three sides are different lengths. Anisosceles triangle
is a triangle with at least two congruent sides. Anequilateral triangleis a triangle with three congruent sides.
Guided Practice
- How many triangles are in the diagram below?
- Classify the triangle by its sides and angles.
- Classify the triangle by its sides and angles.
Answers: