Shapes and Angles

Lesson 18

Math

Unit 8

4th Grade

Lesson 18 of 18

Objective


Identify and draw lines of symmetry in two-dimensional figures.

Common Core Standards


Core Standards

  • 4.G.A.3 — Recognize a line of symmetry for a two-dimensional figure as a line across the figure such that the figure can be folded along the line into matching parts. Identify line-symmetric figures and draw lines of symmetry.

Criteria for Success


  1. Understand that a line of symmetry is a line through a figure such that when the figure is folded along the line, two halves are created that match up exactly.
  2. Understand that a symmetric shape is a shape for which at least one line of symmetry exists.
  3. Identify whether a given line is a line of symmetry or not.
  4. Draw lines of symmetry in a figure.
  5. Understand the special case that circles have an infinite number of lines of symmetry, all of which go through its center.
  6. Given half of a symmetric shape and its line of symmetry on grid paper, draw the whole symmetric shape (MP.6).

Tips for Teachers


Lesson Materials

  • Scissors (1 per student)
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Anchor Tasks

25-30 minutes


Problem 1

Watch the following YouTube video.

All of the following cutout shapes were made the same way, by folding a piece of paper before cutting. Where do you think the paper was folded to make each of the following shapes? Draw a line to show your thinking.

Guiding Questions

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Student Response

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Problem 2

Find the line(s) of symmetry in the following geometric figures, if there are any.

a.   

b.   

c.   

d.   

e.   

Guiding Questions

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Student Response

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Problem 3

a.   Garret drew a rhombus on grid paper, and then folded the paper along the rhombus’ line of symmetry. Draw what the grid paper looks like folded.

            

b.   Felicia drew part of a shape on grid paper. The folded edge is the full shape’s line of symmetry. Draw what the grid paper looks like unfolded.

Guiding Questions

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Student Response

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Problem Set

15-20 minutes


Discussion of Problem Set

  • In #2, which figures had lines of symmetry that were most difficult to see? Why were some easier and others more difficult?
  • In #3, why does a circle have an infinite number of lines of symmetry?
  • In #4, how many lines of symmetry did the equilateral triangle have? The isosceles triangle? The scalene triangle? Do you think that is true of all triangles of those types? Why or why not? 
  • In #5, what method did you use to complete each figure? How would you complete the figure if there were no graph paper?
  • Identify objects around the classroom or in nature that have lines of symmetry.

Target Task

5-10 minutes


Problem 1

Which of the following shows a line of symmetry? 

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Student Response

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Problem 2

Draw as many lines of symmetry as you can find in the figure below. 

Student Response

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Additional Practice


The Extra Practice Problems can be used as additional practice for homework, during an intervention block, etc. Daily Word Problems and Fluency Activities are aligned to the content of the unit but not necessarily to the lesson objective, therefore feel free to use them anytime during your school day.

Word Problems and Fluency Activities

Word Problems and Fluency Activities

Help students strengthen their application and fluency skills with daily word problem practice and content-aligned fluency activities.

Lesson Map

A7CB09C2-D12F-4F55-80DB-37298FF0A765

Topic A: Lines and Angles

Topic B: Measures of Angles

Topic C: Measures of Adjacent Angles

Topic D: Shapes and Lines of Symmetry

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