Take Apart 2D Shapes

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SWBAT decompose two-dimensional shapes into parts.

Big Idea

Let's break it down! In this lesson, students will look at a new shape and decompose it into smaller shapes.

Activating Strategy

10 minutes

I start the lesson by reading aloud, “Not a Box”.  This story shows the many creative ways that the main character uses his imagination to turn a box into something fun! 

This story really engages the students, and I like to stop at each page and point out all the different shapes that are used to make our new shape.  This helps in getting their brains ready for taking apart new shapes.

I  have found that, in working to mastery of 1.G.A.2, some students may have difficulty visualizing how to take apart the composite shape to find the smaller shapes.  To help overcome this, I provide students with Take apart 2D shapes-shape cut outs.docx and allow them to fold the shape until they make the correct new smaller shapes.  This helps them to visualize the smaller shapes that make up the larger shape by using concrete models.

Teaching Strategies

15 minutes

I read the following problem from the Take Apart 2D Shapes.ppt:

Karen put some triangles and rectangles together. She drew pictures to show what she made. Color to show how Karen put the shapes together.

Then I guide children through the solution process by asking questions to the group:

  • How do you know which shapes are rectangles? (They have 4 straight sides and 4 vertices.)

Have children identify the rectangle in both drawings. Encourage them to trace the rectangles with their fingers. Then have children use an orange crayon to outline and color each rectangle.

  • How do you know which shapes are triangles? (They have 3 straight sides and 3 vertices.

Have children identify the triangles the same way, first by tracing them and then by outlining and coloring the triangles purple.)

  • Why are these shapes in both drawings triangles even though they look different? (They all have 3 straight sides and 3 vertices.)
  • How is the first picture like the second picture? (It has the same number of triangles and rectangles.)
  • How is the first picture different from the second picture? (It has 4 sides and the second one has 6 sides.)

As children explore geometric concepts, I have found that it is important that they are able to describe and recognize the attributes of two-dimensional shapes. Children use these skills when they decompose shapes by drawing lines to create new shapes within a given shape (MP7).

  • Help children recognize that each line they draw will be a side in the new shapes they create.
  • Encourage children to step back and look at the whole shape from a different perspective—as a sum of its parts. They can adjust their focus and identify either the single shape or an assemblage of more than one shape.

Independent Practice

30 minutes

Next I guide students through the model on the top of their Take Apart 2D shapes_worksheet.docx and have them trace the line in the circle.  I make sure to explain that this line shows how two shapes can be made from the circle.  Ask the students:

  • If you put the two parts together again, what shape do you make? (a circle)
  • Will you always get the same two shapes if you draw the line in a different place? Why or why not? (No. You can draw a line near the bottom of the circle and you would get a big shape and a small shape.)

I then release students to complete the worksheet on their own.


5 minutes

To close out the lesson, I have students draw a shape in their math journal.  Then they are draw one or two lines to show the parts of the shape.