Classifying Triangles and Relationships in Triangles A triangle is a closed plane figure with three straight sides that connect three points, such as A , B , and C . You read D ABC as “triangle ABC .” D ABC has sides  ,  and  , and vertices A, B , and C . You can classify triangles by their angles of their sides.
Classifying Triangles by Angles |
An acute triangle has three acute angles.
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A right triangle has one right angle.
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An obtuse triangle has one obtuse angle.
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Example: Classify the triangle by its angles.
Solution: a. The triangle is obtuse because it has 1 obtuse angle.
b. The triangle is acute because it has 3 acute angles.
c. The triangle is right because it has 1 right angle.
Side of a Triangle: You can use special marks on a drawing to indicate that two sides have the same length as shown at the right.
Classifying Triangles by Sides |
An equilateral triangle has three sides of the same length.
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An isosceles triangle has at least two sides of the same length.
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A scalene triangle ahs three sides of different lengths.
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Equilateral Triangle or Equiangular Triangle
Isosceles Triangle
Scalene Triangle
Example: Classify the triangle by its sides.
Solution: a. The triangle is scalene because all of its sides have different lengths.
b. The triangle is equilateral because all of sides have the same length.
c. The triangle is isosceles because two of its sides have the same length.
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