Book Lesson

Classifying Triangles:

About Triangles:

A triangle is a polygon with 3 sides.
Any triangle you can create has at least one side length and one angle measure classification.

What are the different parts of a triangle?

Base: Any side of a triangle can be called the base.
Vertex: The different endpoints of the triangle. To name a triangle or any other figure, name the vertices in either clockwise or counterclockwise order.
Height: The line segment that starts at a vertex & is perpendicular to the base is called the height or altitude.

How do we identify different types of triangles?

Triangles can be classified by their side lengths & angles. Triangles can have more than one type of classification.

What are the different types of triangles?

Side Classification:
Scalene: No equal sides or equal angles.
Isosceles: At least two equal sides and at least two equal angles.
Equilateral: Three equal sides and three equal angles.

Angle Classification:
Right: has a right angle (measure 90°).
Acute: All angles measure less than 90°.
Obtuse: Has an angle that measures more than 90°.

How do we find the missing interior angle of a triangle?

The sum of the interior angles of ANY triangle is 180°.

Step 1: Add the 2 given angles of the triangle.

Step 2: Subtract the sum of the 2 angles from 180.

This will give you the final angle.

Types of Triangles

external image Traingles.jpg

Congruent Triangles

What are congruent triangles?

Congruent figures are shapes that have the same shape and size.

When two figures are congruent – corresponding sides and corresponding angles are all congruent (or the same).

Congruent triangles will fit exactly on top of each other.

Similar Triangles

What are similar triangles?

Two figures are similar when they have the same shape.

Similar figures can be the same size or different sizes.

When two figures are similar, the ratios of lengths of their corresponding sides are equal.

The corresponding angles of similar figures are congruent.

How do we determine if two triangles are similar?

Step 1: Check the ratio of corresponding sides.

Step 2: Look at the arcs of corresponding angles in the two triangles – they should be congruent.

Classify triangles by sides:

Classify triangles by angles:


(give students white boards and have them hold up their answer for each one)