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A triangle is a shape with 3 sides and 3 angles.

You can classify triangles by either their angles or their sides, as you will soon see.

By their Sides

The symbol “|” is used to identify congruent sides of a triangle.

A scalene triangle has no equal sides. They are all different lengths.

An isosceles triangle has 2 equal sides, but the third one isn’t the same length as the first two. The short lines intersecting the sides signal that those sides are equal in length.

An equilateral triangle has all 3 sides the same length. You can almost see the word “equal” in the name, referring to “equal sides.” The short lines intersecting the sides of the triangle indicate that all the sides are the same length.

By their Angles

The symbol “|” is used to identify congruent sides of a triangle. The symbol, “ ” indicates congruent angles of a triangle. The symbol “ ” indicates a right angle.

If all the angles in a triangle are acute (less than 90°), the triangle is an acute triangle.

If any of the angles in a triangle are obtuse (measuring more than 90°), then the triangle is classified as an Obtuse Triangle.

If any of the angles in a triangle measure 90° (a right angle), then the triangle is a right triangle. The little box in the corner indicates that that angle of the triangle is the right angle.

PRACTICE

Triangle ATriangle B Angles- Sides- Angles- Sides- Obtuse Scalene Acute Equilateral

Triangle CTriangle D Angles- Sides- Angles- Sides- Right Scalene Acute Isosceles

Triangle ETriangle F Angles- Sides- Angles- Sides- Acute Scalene Right Isosceles

Symbol ASymbol B What it means- Equal-length sides Right angle

Symbol C What it means- Congruent angles