Warm Up

1. A _________________ is a transformation that shifts or slides every point of a figure or graph the same distance in the same direction.

2. Describe the transformation shown.

3. Which equation gives a reflection of over the y-axis?

Lesson 76, Symmetry

Geometry

Vocabulary

• Symmetry: a property illustrated when the image of a transformation coincides with the preimage

• Line symmetry: image coincides with preimage after a reflection across a line

• Line of Symmetry: line that divides the figure into two congruent, reflected halves

Vocabulary

• Rotational Symmetry: image coincides with preimage after a rotation of less than 360 degrees

• Angle of rotational symmetry: smallest angle through which a figure can be rotated in order to coincide with itself

• Order of rotational symmetry: number of times a figure with rotational symmetry coincides with itself as it rotates 360 degrees• Order is equal to divided by the angle of rotational symmetry

Example

Identify whether each figure has a line of symmetry. If it does, draw the line of symmetry.

Example

Triangle ABC has vertices A(0,0), B(2,0) and C(2,3). If the triangle is reflected over the x-axis, will the first triangle and the reflected triangle have a line of symmetry? If so, where is the line?

What is the order of rotational symmetry of the star? What is the angle of rotational symmetry?

Example

Tiles often have many lines of symmetry so they can fit easily into patterns. How many lines of symmetry does each regular polygon with up to ten sides have?

Homework Questions

Correcting Homework

• Get your homework out so I can stamp it.

• If you do not have your homework complete, write your name on the board.

• Use red pen/pencil to correct.

• Trade with someone who has the homework done.

• If you didn’t get it done, you need to be working on finishing it now. Remember no cheating, and no work = no credit.

• Make sure you write CB with your name or you will lose your citizenship points for today.

Homework

• 1st and 2nd hour: Lesson 76: a-d, 2, 3, 5, 7, 9, 11, 14, 17, 19, 21, 22, 24, 26, 30

• 3rd hour: Lesson 76: a-d, 2, 3, 5, 7, 9, 14, 17, 19, 21, 24, 30