Lesson 5 Some Triangles, All Triangles

    • Let’s explore what makes a shape a triangle.

Warm-up Which One Doesn’t Belong: Shapes

Which one doesn’t belong?

  1. Closed shape. 4 curved equal length sides. 4 points.
  2. 2 triangles, touching at one point.
  3. Closed shape. 3 straight sides. 1 square corner.
  4. Open shape, 3 corners.

Activity 1 Triangles and “Not Triangles”

  1. These are triangles.

    Closed shape. 3 straight sides. 3 corners, pointy.

    What is the same about all these triangles?

  2. These are not triangles.

    Closed shape. 4 straight sides, different lengths. 4 points.
    Shape. 3 straight sides that do not touch.
    Closed shape. 3 total sides, 2 straight, 1 curved.

    What makes these shapes different from the triangles?

Activity 2 Draw Triangles

  1. Draw 3 different triangles.

    Dot paper.
  2. Draw 3 different shapes that are not triangles.

    Dot paper.

Activity 3 Centers: Choice Time

Choose a center.


How Are They the Same?

Center. How Are They the Same.


Center. Compare.

How Close?

Center. How Close.

Practice Problem

Problem 1

Is this shape a triangle? Why or why not?

Dot paper drawing. Shape with 4 sides.