MA.912.GR.2.4

Benchmark Instructional Guide

Connecting Benchmarks/Horizontal Alignment

• MA.912.AR.2.5 
• MA.912.AR.5.6 
• MA.912.GR.1.6

Terms from the K-12 Glossary

• Reflection 
• Rotation 
• Translation

Vertical Alignment

Previous Benchmarks

• MA.912.GR.2.1
• MA.912.GR.2.2
• MA.912.GR.2.3
• MA.912.GR.2.5

Next Benchmarks

• MA.912.T.2 
• MA.912.T.3

Purpose and Instructional Strategies

• Instruction includes multiple opportunities for students to explore symmetries using both physical exploration (transparencies, mirrors or patty paper) and virtual exploration, when possible. 
• Instruction includes using a variety of shapes (mathematical and real world) to explore the reflection symmetry and rotational symmetry of the shapes. Include identifying the lines of symmetry, the order of symmetry and the angle of rotation that will map the figure onto itself. 
• The order of symmetry is the smallest (nonzero) number of times that you must apply the corresponding transformation to map each point of the figure onto itself. 
• The order of rotational symmetry is the number of times the figure maps onto itself as it rotates through 360° about the figure’s center. Instruction includes identifying the angles of rotation when determining symmetries of rotation. 
  • For example, the order of rotational symmetry for a regular hexagon is 6 with the angle of rotation of 60°. 
  • For example, the order of rotational symmetry for an isosceles trapezoid is 0. 
  • For example, the figure below has an order of 4 with angle of rotation of 90°. 
    
    
    

• The order of a translational symmetry is infinite because no matter how many times one applies it, no point gets mapped onto itself. Translational symmetry results from mapping a figure onto itself by moving it a certain distance in a certain direction. Show students tessellations (covering of a plane using one or more geometric shapes with no overlaps and no gaps), or have them create them, and discuss the translational symmetry in the tessellation (MTR.5.1). 
• Depending on a student’s pathway, instruction includes making the connection to symmetry as a key feature of the graphs of polynomial and trigonometric functions and the importance of applying such functions in the real world. 
• Problem types include using symmetries to classify geometric figures. 
  • For example, given that a two-dimensional figure has one line of symmetry through midpoints of the two parallel sides, no rotational symmetry and no point symmetry, students can deduce that this figure is a trapezoid.

Common Misconceptions or Errors

• Students may not make the connections to the idea of an infinite pattern (wallpaper pattern, border pattern, etc.) when working with only a portion of that pattern. If students arrive at an order for a translational symmetry that is not infinite, ask them if their answer would be different if the pattern continued. 
• Students may have difficulty distinguishing between mapping a figure onto itself and mapping every point of the figure onto itself. To help address this misconception, have students highlight a particular point and observe how it is affected by one application of a transformation that maps the figure onto itself.

Instructional Tasks

• Use the figures below to answer the questions. 

• Part A. Draw the lines of symmetry, if any, on each figure. 
• Part B. What is the order of symmetry for reflections? 
• Part C. Determine the order of rotational symmetry for each figure. Discuss with a partner how you determined each. 

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