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2D shapes Regular polygons Irregular polygons Parallel lines Perpendicular linesHere you will learn about lines of symmetry, including symmetry properties within polygons, angle properties, and symmetry of different line graphs.

Students first learn about line symmetry in grade 4 with their work with 2D shapes in geometry.

**Lines of symmetry** are straight lines that divide a shape into two equal parts, where one part is an exact reflection or mirror image of the other.

In geometry, many 2D shapes have line symmetry.

For example,

Equilateral triangle | Square | Rectangle | Regular hexagon |
---|---|---|---|

3 lines of symmetry | 4 lines of symmetry | 2 lines of symmetry | 6 lines of symmetry |

Line symmetry can be seen in nature.

For example,

Line symmetry can be seen in art.

For example,

**Regular polygons** are polygons that have equal side lengths and equal angle measures.

For example,

Equilateral Triangle |
---|

3 lines of symmetry Notice how each line of symmetry can be a fold line. |

Square |

4 lines of symmetry Notice how each line of symmetry can be a fold line. |

Regular pentagon |

5 lines of symmetry Notice how each line of symmetry can be a fold line. |

Regular hexagon |

6 lines of symmetry Notice how each line of symmetry can be a fold line. |

Notice how the number of lines of symmetry in these regular polygons is equal to the number of sides and angles the polygons have.

All regular polygons share the property that the **number of sides is equal to the number of lines of symmetry.**

Use this quiz to check your grade 2 – 4 students’ understanding of 2D shape. 10+ questions with answers covering a range of 2nd, 3rd and 4th grade 2D shape topics to identify areas of strength and support!

DOWNLOAD FREEUse this quiz to check your grade 2 – 4 students’ understanding of 2D shape. 10+ questions with answers covering a range of 2nd, 3rd and 4th grade 2D shape topics to identify areas of strength and support!

DOWNLOAD FREEIrregular polygons are polygons that do not have all equal angles and all equal side lengths. However, they can have line symmetry.

For example, these irregular polygons have line symmetry.

Let’s look at different shapes (specifically quadrilaterals) and their number of lines of symmetry.

Quadrilateral | Image | Number of Linesof Symmetry |
---|---|---|

Square | 4 | |

Rectangle | 2 | |

Parallelogram | 0 | |

Rhombus | 2 | |

Kite | 1 | |

Trapezoid | 0 | |

Isosceles Trapezoid | 1 | |

Irregular | 0 |

The diameter of a circle splits the circle into two equal sized half circles, or semicircles. Think of the diameter as the line of reflection.

A circle can be folded in-half many ways, meaning that there are an infinite number of lines of symmetry.

How does this apply to 4th grade math?

**Grade 4: Geometry (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.

In order to draw/identify lines of symmetry:

**Locate the center of the 2D shape.****Draw a vertical line through the center and check for line symmetry.****Draw a horizontal line through the marked center and check for line symmetry.****Draw a line from each vertex through the center and check for symmetry.****State the number of lines of symmetry.**

How many lines of symmetry does the equilateral triangle have?

**Locate the center of the 2D shape.**

2**Draw a vertical line through the center and check for line symmetry.**

The vertical line through the center is a line of symmetry dividing the triangle into two equal halves that are mirror images of each other. Notice that it can be a fold line.

3**Draw a horizontal line through the marked center and check for line symmetry.**

4**Draw a line from each vertex through the center and check for line symmetry.**

5**State the number of lines of symmetry.**

The equilateral triangle has 3 lines of symmetry. An equilateral triangle has 3 equal sides and 3 equal angles, so it will have 3 lines of symmetry.

How many lines of symmetry does the regular pentagon have?

**Locate the center of the 2D shape.**

**Draw a vertical line through the center and check for line symmetry.**

The vertical line is a line of symmetry because it divides the regular pentagon into two equal halves that are mirror images of each other.

**Draw a horizontal line through the marked center and check for line symmetry.**

The horizontal line is NOT a line of symmetry because it does not divide the regular pentagon into two equal halves that are mirror images of each other.

**Draw a line from each vertex through the center and check for line symmetry. **

**State the number of lines of symmetry.**

A regular pentagon has 5 lines of symmetry. A regular pentagon has 5 equal sides and 5 equal angles, so it will have 5 lines of symmetry.

How many lines of symmetry does the trapezoid have?

**Locate the center of the 2D shape.**

**Draw a vertical line through the center and check for line symmetry.**

The vertical line is a line of symmetry because it divides the trapezoid into two equal halves that are mirror images of each other.

**Draw a horizontal line through the marked center and check for line symmetry.**

The horizontal line is NOT a line of symmetry because it does not divide the trapezoid into two equal halves that are mirror images of each other.

**Draw a line from each vertex through the center and check for line symmetry. **

**State the number of lines of symmetry.**

The trapezoid has one line of symmetry.

How many lines of symmetry does the irregular hexagon have?

**Locate the center of the 2D shape.**

**Draw a vertical line through the center and check for line symmetry.**

The vertical line is a line of symmetry because it divides the irregular hexagon into two equal halves that are mirror images.

**Draw a horizontal line through the marked center and check for line symmetry.**

The horizontal line is a line of symmetry because it divides the irregular hexagon into two equal halves that are mirror images.

**Draw a line from each vertex through the center and check for line symmetry. **

**State the number of lines of symmetry.**

The irregular hexagon has two lines of symmetry.

How many lines of symmetry does the parallelogram have?

**Locate the center of the 2D shape.**

**Draw a vertical line through the center and check for line symmetry.**

The vertical line is NOT a line of symmetry because it does not divide the parallelogram into two equal halves that are mirror images of each other.

**Draw a horizontal line through the marked center and check for line symmetry.**

The horizontal line is NOT a line of symmetry because it does not divide the parallelogram into two equal halves that are mirror images of each other.

**Draw a line from each vertex through the center and check for line symmetry. **

**State the number of lines of symmetry.**

The parallelogram does not have any lines of symmetry.

How many lines of symmetry does the shape have?

**Locate the center of the 2D shape.**

**Draw a vertical line through the center and check for line symmetry.**

The vertical line is a line of symmetry because it divides the shape into two equal halves that are mirror images of each other.

**Draw a horizontal line through the marked center and check for line symmetry.**

The horizontal line is a line of symmetry because it divides the shape into two equal halves that are mirror images of each other.

**Draw a line from each vertex through the center and check for line symmetry. **

**State the number of lines of symmetry.**

The irregular shape has 4 lines of symmetry.

- Use hands on manipulatives to help students gain understanding of lines of symmetry.

- Infuse creative activities such as origami so that students can build understanding through fun learning tasks.

**Not visualizing that a line of symmetry is like a fold line**

For example, thinking that this is a line of symmetry. If you were to fold the parallelogram along the line of symmetry, the two halves would not be mirror images of each other.

**Thinking that all quadrilaterals have**\bf{4}**lines of symmetry**

The only quadrilateral with 4 lines of symmetry is a square because it is a regular polygon. The other quadrilaterals are not regular polygons so do not have 4 lines of symmetry.

**The number of sides always**\textbf{=}**the number of lines of symmetry**

Although this is true for regular shapes, it is not true for all shapes.

**Thinking all triangles have**\bf{3}**lines of symmetry**

For example, this right scalene triangle does not have any lines of symmetry.

Equilateral triangles are the only triangles that have 3 lines of symmetry because it is a regular triangle.

1. Find the number of lines of symmetry for the regular hexagon below.

4

2

6

3

Regular polygons have the same number of lines of symmetry as the number of sides that they have.

A hexagon has 6 sides so it has 6 lines of symmetry.

Marking the center, there are 6 lines that can be drawn through the center that cut the regular hexagon into equal halves that are mirror images of each other.

2. Find the number of lines of symmetry for the rectangle below.

1

2

0

4

After marking the center, the vertical line through the center and the horizontal line through the center are the only lines of symmetry.

Any other line drawn through the center will not result in the shape being divided into two equal halves that are mirror images.

3. Find the number of lines of symmetry for the kite below.

0

4

2

1

After marking the center, the only line of symmetry is the horizontal line.

There is no other line that can be drawn through the center that will divide the shape into two equal parts that are mirror images of each other.

4. Find the number of lines of symmetry for the irregular hexagon below.

2

0

3

6

After marking the center, the lines of symmetry are the vertical line through the center and the horizontal line through the center.

No other lines through the center will divide the shape into two equal parts that are mirror images of each other.

5. How many lines of symmetry does the smiley have?

0

1

infinite

8

Circles have an infinite amount of lines of symmetry. However, you have to take the eyes and mouth into account when figuring out the lines of symmetry.

There is only one line of symmetry vertically down the face that will divide the smiley face into two equal mirror images of each other.

6. How many lines of symmetry does the photo have?

0

1

2

infinite

The photo has two lines of symmetry, one that is horizontal through the center and the other that is vertical through the center.

There are no other lines that can be drawn that divides the photo into two equal halves that are mirror images of each other.

There are two types of symmetry. Line symmetry is drawing a line through the center of a figure where both halves are mirror images of each other. Rotational symmetry involves turning a shape and determining how many times the shape appears as its original in one complete 360^{\circ} turn.

In algebra, you will learn about quadratic functions whose graph is a parabola. The parabola has a line going through the center that divides it into identical halves, which is the axis of symmetry. It is also the line of symmetry of the parabola.

No, a scalene triangle does not have any lines of symmetry. Meaning there are no imaginary lines or mirror lines that can be drawn through the center of the triangle, where one half is equal and the mirror image of the other half. Isosceles triangles have only one line of symmetry. Equilateral triangles are regular polygons, so they have 3 lines of symmetry.

Yes, regular polygons are symmetrical shapes because they have equal sides and equal angles. They have line symmetry and rotational symmetry.

Congruent means identical in shape and size. When drawing a dotted line representing line symmetry, each half of the shape is congruent.

- Angles in polygons
- Congruence and similarity
- Transformations

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