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HSG.CO.A.3Common CoreMathGeometryGrades 9-12

HSG.CO.A.3: Rotations and Reflections That Carry a Polygon onto Itself

In plain English: HSG.CO.A.3 is the Common Core geometry standard that asks students to describe the rotations and reflections that carry a rectangle, parallelogram, trapezoid or regular polygon onto itself. Students name each line of reflection and each center and angle of rotation, and explain why other lines and angles fail. It is usually taught in high school Geometry.

Given a rectangle, parallelogram, trapezoid, or regular polygon, describe the rotations and reflections that carry it onto itself.

Common Core State Standards for Mathematics · Domain: Congruence (CO) · Cluster: Experiment with transformations in the plane
Also written as HSG-CO.A.3 or G-CO.3 · Official standard

01

Lesson Plan

65-70 min

Overview

Students find every rotation and reflection that carries a figure onto itself, meaning that after the motion the figure occupies exactly the same set of points it did before. They work through the four families the standard names: rectangles, parallelograms, trapezoids and regular polygons. For each figure they name the lines of reflection and the center and angles of rotation, and they explain why lines and angles that look plausible, such as the diagonal of a rectangle, do not work.

Students test symmetries three ways: by turning and flipping tracing paper, by checking vertex coordinates under rules such as (x, y) → (-x, y), and by using geometry software. The lesson ends with the pattern for regular polygons: a regular polygon with n sides has n lines of reflection and is carried onto itself by rotations of 360°/n and its multiples.

Learning Objectives

By the end of this lesson, students will be able to:

  • Describe every rotation and reflection that carries a given rectangle, parallelogram, trapezoid or regular polygon onto itself
  • Locate the center of rotation and each line of reflection precisely, for example as the intersection of the diagonals or the line through the midpoints of opposite sides
  • Explain with a counterexample why a proposed line or angle does not carry a figure onto itself
  • Verify a symmetry with coordinates by showing that the set of vertices maps onto itself
  • State and use the rotation angles 360°/n and the n lines of reflection of a regular n-gon

Prior Knowledge Required

Students should already be comfortable with:

  • Recognizing and drawing lines of symmetry 4.G.A.3
  • Properties of rotations and reflections verified with tracing paper 8.G.A.1
  • Transformations as functions on points and coordinate rules HSG.CO.A.2
  • Classifying quadrilaterals in a hierarchy, for example every square is a rectangle and a rhombus 5.G.B.4
  • The midpoint of a segment in the coordinate plane

Lesson Procedure

65-70 minutes of class time across 5 phases.

  1. Warm-Up10 minutes

    Give each student a paper rectangle that is not a square (for example 10 cm by 5 cm) with one corner marked with a dot on the front.

    Warm-Up Prompt

    "Find every way to pick up the rectangle and put it back so that it fits its outline exactly. You may turn it or flip it. For each way, where does the dotted corner end up?"

    Students find four positions: the original, a half turn, a flip over the long direction and a flip over the short direction. The dot visits each corner exactly once, which is a useful check that the list is complete. Many students also try folding along a diagonal; let them fold and see that the corners do not match. Introduce the phrase "carries the figure onto itself".

  2. Direct Instruction20 minutes

    Define: a rigid motion carries a figure onto itself when the image is exactly the same set of points as the figure. For a polygon, it is enough to check that the vertices land on vertices and that sides go to sides. Every figure is carried onto itself by the 360° rotation (the identity), so the interesting question is what else works. Use Diagram 1 and Diagram 2 with these examples:

    • Rectangle

      Rectangle with vertices (-4, -2), (4, -2), (4, 2), (-4, 2). Test the axes, the 180° rotation about the origin and the diagonal through (-4, -2) and (4, 2).

      Equation: Reflections across y = 0 and x = 0 and the 180° rotation work; reflecting (4, -2) across the diagonal gives (0.8, 4.4), not a vertex

    • Parallelogram

      Parallelogram with vertices (0, 0), (6, 0), (8, 3), (2, 3). Its diagonals meet at (4, 1.5).

      Equation: 180° rotation about (4, 1.5): (x, y) → (8 - x, 3 - y) sends (0, 0) to (8, 3) and (6, 0) to (2, 3); no reflection works

    • Isosceles trapezoid

      Trapezoid with vertices (-4, 0), (4, 0), (2, 3), (-2, 3), legs of equal length.

      Equation: Reflection across x = 0 works; the 180° rotation about (0, 1.5) sends (-4, 0) to (4, 3), not a vertex

    • Regular hexagon

      Rotations about the center and lines of reflection.

      Equation: Rotations of 60°, 120°, 180°, 240°, 300° (360°/6 = 60°) and 6 lines of reflection

    • Regular pentagon

      Rotations about the center and lines of reflection.

      Equation: Rotations of 72°, 144°, 216°, 288° (360°/5 = 72°) and 5 lines, each through a vertex and the midpoint of the opposite side

    Stress three points. First, the center of rotation for a rectangle or parallelogram is the point where the diagonals intersect. Second, a general trapezoid has no symmetry except the identity; only an isosceles trapezoid has a line of reflection. Some textbooks define a trapezoid as having at least one pair of parallel sides, which makes every parallelogram a trapezoid; say which definition your course uses. Third, for a regular polygon with an even number of sides, half the lines pass through opposite vertices and half through midpoints of opposite sides; with an odd number, every line passes through a vertex and the midpoint of the opposite side.

  3. Guided Practice15-20 minutes

    Pairs describe all the symmetries of three figures and verify one reflection and one rotation with coordinates:

    1. Square with vertices (0, 0), (4, 0), (4, 4), (0, 4): rotations of 90°, 180° and 270° about (2, 2), and 4 lines of reflection: x = 2, y = 2 and both diagonals. A square is a regular polygon, so it has 4 lines and rotations by multiples of 90°.
    2. Rhombus with vertices (0, 3), (4, 0), (0, -3), (-4, 0): the 180° rotation about the origin and reflections across both diagonals, which here are the axes. A rhombus is a parallelogram, and it is the kind of parallelogram that does have lines of reflection.
    3. Trapezoid with vertices (-3, 0), (5, 0), (3, 2), (0, 2): the legs have lengths √13 and √8, so it is not isosceles and only the 360° rotation carries it onto itself.

    Ask each pair to write its evidence as a mapping of vertices, for example "(0, 3) → (0, -3)". Listen for students who check only one vertex; all of them must land on vertices.

  4. Independent Practice15 minutes

    Students work alone on two tasks. (1) Rectangle with vertices (0, 0), (10, 0), (10, 4), (0, 4): give the equations of both lines of reflection, the center of the 180° rotation, and the vertex mapping for each. (Answer: x = 5 and y = 2, center (5, 2).) (2) Regular heptagon: list the rotation angles less than 360° to the nearest tenth of a degree, and describe the 7 lines of reflection. (Answer: multiples of 360°/7 ≈ 51.4°: 51.4°, 102.9°, 154.3°, 205.7°, 257.1°, 308.6°; each line passes through a vertex and the midpoint of the opposite side.)

  5. Closure5 minutes

    Exit ticket: (1) Name a quadrilateral that is carried onto itself by a 180° rotation but by no reflection. (A parallelogram that is not a rectangle or a rhombus.) (2) Name a quadrilateral with a line of reflection but no rotation other than 360°. (An isosceles trapezoid.) (3) What is the smallest positive rotation that carries a regular polygon with 20 sides onto itself, and how many lines of reflection does it have? (18°, and 20 lines.)

Differentiation Strategies

For Struggling Students

  • Have students number the vertices of a cut-out figure on both sides so they can see where each vertex goes after a turn or flip
  • Provide a recording table with columns for the motion, its center or line, and the vertex mapping
  • Start with the rectangle and the square before the parallelogram and the trapezoid

For Advanced Students

  • Ask students to prove that if a quadrilateral is carried onto itself by a 180° rotation about a point, then it is a parallelogram
  • Ask students to count all the symmetries of a regular n-gon (rotations including 360° plus reflections) and explain why the total is 2n
  • Ask whether a figure can have exactly two lines of reflection that are not perpendicular, and to justify the answer

Assessment Guidance

What to Look For

Complete descriptions name the center and angle of every rotation and give each line of reflection precisely (an equation, or "the line through the midpoints of AB and CD"), not just "it has symmetry". Check that students reject the diagonals of a non-square rectangle and do not claim reflections for a general parallelogram. For regular polygons, look for the angle 360°/n and all its multiples below 360°, and for the correct description of where the lines pass for odd and even n.

02

Classroom Activities

3 Activities

1

Trace, Turn and Flip

20 minPairs

Pairs use tracing paper and a pushpin to find every rotation and reflection that carries four quadrilaterals onto themselves, then compare the results.

Materials per Pair

  • 4 cut-out quadrilaterals: a rectangle that is not a square, a parallelogram with no right angles and unequal adjacent sides, an isosceles trapezoid, and a trapezoid that is not isosceles
  • Tracing paper, a pushpin and a ruler

Procedure

  • Trace a figure, then look for a point where you can pin the tracing and turn it so it fits the figure again; record the center and the angle
  • Look for lines where you can flip the tracing over so it fits again; draw each line on the figure
  • Mark one vertex with a dot and record where it goes under every motion you find
  • Complete a class chart with the number of rotations (including 360°) and the number of reflections for each figure

Discussion Questions

  • Which figures have a line of reflection but no half-turn? Which have a half-turn but no line of reflection?
  • Why can a pin placed anywhere other than the intersection of the diagonals never work for the rectangle?
2

Regular Polygon Pattern Hunt

20 minGroups of 3-4

Groups use pattern blocks or printed regular polygons with 3, 4, 5 and 6 sides to find the rotations and lines of reflection of each, then look for a rule that predicts them for any number of sides.

Procedure

  • Trace each polygon, mark its center, and turn the tracing until the polygon fits again; record the smallest angle that works
  • Fold paper copies to find every line of reflection and sketch where each line crosses the polygon: through a vertex, through the midpoint of a side, or both
  • Fill in a table with the number of sides n, the smallest rotation angle and the number of lines
  • Use the pattern to predict the smallest rotation and the number of lines for a regular polygon with 24 sides (15° and 24 lines), and justify the prediction

Challenge Variation

Groups explain why the lines of reflection look different for odd and even n, using the idea that a line through the center must reach either a vertex or the midpoint of a side on the far side.

3

Symmetry Check with Coordinates and Software

20 minPairs

Pairs place three quadrilaterals on a coordinate grid in geometry software, apply the reflect and rotate tools, and confirm each symmetry by checking that every vertex lands on a vertex.

The 3 Figures

  • Rectangle (1, 1), (7, 1), (7, 4), (1, 4): lines x = 4 and y = 2.5, and 180° about (4, 2.5)
  • Parallelogram (0, 0), (5, 0), (7, 3), (2, 3): 180° about (3.5, 1.5) only
  • Isosceles trapezoid (1, 0), (7, 0), (6, 3), (2, 3): the line x = 4 only

Procedure

  • Construct each figure and apply the motion listed; check that the image covers the figure exactly
  • Try one motion that fails for each figure and record the vertex that lands off the figure
  • Write each successful symmetry as a coordinate rule, for example (x, y) → (8 - x, y) for the reflection across x = 4

Modification for Distance Learning

Share one geometry software file with the three figures already built; pairs record their vertex mappings in a shared table.

03

Diagrams & Visual Aids

2 diagrams

Diagram 1: Symmetries of a Rectangle, a Parallelogram and an Isosceles Trapezoid

Rectangle 2 lines of reflection 180° rotation about the center A diagonal is not a line of reflection Parallelogram No lines of reflection 180° rotation about the point where the diagonals meet 180° Isosceles trapezoid 1 line of reflection through the midpoints of the bases No rotation except 360°
Drawn to scale from the coordinates in the worked examples. Dashed blue lines are lines of reflection and the blue dot is a center of rotation. The dotted red diagonal of the rectangle is not a line of reflection, and the parallelogram has only its 180° rotation about the intersection of its diagonals.

Diagram 2: Rotations and Lines of Reflection of Regular Polygons

Regular hexagon 60° 6 lines: 3 through opposite vertices, 3 through midpoints of opposite sides Rotations: 60°, 120°, 180°, 240°, 300° Regular pentagon 72° 5 lines, each through a vertex and the midpoint of the opposite side Rotations: 72°, 144°, 216°, 288°
A regular hexagon has 6 lines of reflection and is carried onto itself by rotations of 60° and its multiples about the center. A regular pentagon has 5 lines, each through a vertex and the midpoint of the opposite side, and rotations of 72° and its multiples.

04

Homework Assignment

~30 min

HSG.CO.A.3 Homework: Rotations and Reflections That Carry a Figure onto Itself

Directions: For every figure, describe each rotation (center and angle) and each reflection (the line) that carries it onto itself, not counting the 360° rotation unless asked. Support each answer with a vertex mapping, and use a specific vertex to show why a proposed motion fails.

Part 1: Rectangles, Parallelograms and Trapezoids (Problems 1-3)

  1. A rectangle has vertices (-6, -3), (6, -3), (6, 3) and (-6, 3). (a) Describe every rotation and reflection that carries it onto itself, with a coordinate rule for each. (b) Show that reflecting (6, -3) across the diagonal through (-6, -3) and (6, 3) does not give a vertex.
  2. A parallelogram has vertices (-1, -2), (5, -2), (7, 2) and (1, 2). (a) Find the center of the 180° rotation that carries it onto itself and show where each vertex goes. (b) Show that the reflection across the vertical line x = 3 does not carry it onto itself.
  3. Compare two trapezoids: the isosceles trapezoid (-6, 0), (6, 0), (3, 5), (-3, 5) and the right trapezoid (0, 0), (8, 0), (5, 4), (0, 4). Describe the rotations and reflections that carry each onto itself and explain the difference.

Part 2: Regular Polygons (Problems 4-6)

  1. A regular octagon is centered at point O. (a) List every rotation about O less than 360° that carries it onto itself. (b) How many lines of reflection does it have, and where do they pass? (c) If vertex 1 is rotated 135° counterclockwise, which vertex does it land on (vertices numbered counterclockwise)?
  2. (a) Describe all the rotations and reflections that carry an equilateral triangle onto itself. (b) A regular polygon has 36 sides. What is the smallest positive rotation that carries it onto itself, and how many lines of reflection does it have?
  3. Make a table for a square, a rhombus that is not a square, a rectangle that is not a square, a parallelogram with no right angles and unequal adjacent sides, an isosceles trapezoid, and a regular polygon with n sides. For each, give the rotation angles less than 360° and the number of lines of reflection.

Rubric

CriterionFull Credit (2 pts)Partial Credit (1 pt)No Credit (0 pts)
Complete ListEvery rotation and reflection found, none extraOne motion missing or one extraSeveral missing or wrong
Precise DescriptionCenters, angles and lines stated exactlyDescribed loosely ("the middle line")Not described
Coordinate EvidenceCorrect vertex mappings and counterexamplesMappings with an errorNo evidence
Regular Polygon PatternUses 360°/n and n lines correctlyPattern partly correctPattern missing or wrong

05

Quiz: 20 Questions

Interactive, with answers

Instructions

Pick an option on each multiple-choice question to see the explanation; tracing paper helps. Work the short answers on paper, then compare with the answer. Reset quiz clears your choices.

Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.

0 of 20 answered · 0 correct

  1. Question 1 of 20 · Multiple Choice

    Which rotation carries a rectangle that is not a square onto itself?

  2. Question 2 of 20 · Multiple Choice

    How many lines of reflection does a rectangle that is not a square have?

  3. Question 3 of 20 · Multiple Choice

    A parallelogram has no right angles and its adjacent sides have different lengths. Which motions, other than the 360° rotation, carry it onto itself?

  4. Question 4 of 20 · Multiple Choice

    A parallelogram has vertices (1, 1), (7, 1), (9, 5) and (3, 5). What is the center of the 180° rotation that carries it onto itself?

  5. Question 5 of 20 · Multiple Choice

    Which line carries an isosceles trapezoid onto itself?

  6. Question 6 of 20 · Multiple Choice

    A trapezoid has vertices (0, 0), (6, 0), (4, 3) and (1, 3). Which motions carry it onto itself?

  7. Question 7 of 20 · Multiple Choice

    What is the smallest positive rotation that carries a regular decagon (10 sides) onto itself?

  8. Question 8 of 20 · Multiple Choice

    How many lines of reflection does a regular polygon with 16 sides have?

  9. Question 9 of 20 · Multiple Choice

    Which rotation about its center carries a regular nonagon (9 sides) onto itself?

  10. Question 10 of 20 · Multiple Choice

    In a regular polygon with an odd number of sides, where does each line of reflection pass?

  11. Question 11 of 20 · Multiple Choice

    A square has vertices (3, 3), (-3, 3), (-3, -3) and (3, -3). Which motion does NOT carry it onto itself?

  12. Question 12 of 20 · Multiple Choice

    A rectangle has vertices (-5, -1), (5, -1), (5, 3) and (-5, 3). Which line carries it onto itself?

  13. Question 13 of 20 · Multiple Choice

    A parallelogram has vertices (0, 0), (5, 0), (8, 4) and (3, 4), and all four sides have length 5. Which line carries it onto itself?

  14. Question 14 of 20 · Multiple Choice

    The smallest positive rotation that carries a regular polygon onto itself is 20°. How many sides does it have?

  15. Question 15 of 20 · Short Answer

    A rectangle measures 10 cm by 6 cm. Describe every rotation and reflection that carries it onto itself, and explain why a diagonal is not a line of reflection.

  16. Question 16 of 20 · Short Answer

    A parallelogram has vertices A(-2, 0), B(4, 0), C(6, 4) and D(0, 4). Find the center of the rotation that carries it onto itself and show where each vertex goes.

  17. Question 17 of 20 · Short Answer

    An isosceles trapezoid has vertices (-5, 0), (5, 0), (3, 4) and (-3, 4). Describe the reflection that carries it onto itself and show it with coordinates. Why does a 180° rotation not work?

  18. Question 18 of 20 · Short Answer

    Describe every rotation less than 360° and every reflection that carries a regular dodecagon (12 sides) onto itself.

  19. Question 19 of 20 · Short Answer

    The smallest positive rotation that carries a regular polygon onto itself is 24°. How many sides does it have, how many lines of reflection, and where do the lines pass?

  20. Question 20 of 20 · Short Answer

    Counting the 360° rotation, how many rotations and reflections carry each figure onto itself: a parallelogram with no right angles and unequal adjacent sides, an isosceles trapezoid, a rectangle that is not a square, and a regular hexagon? Order them from fewest to most.

0 of 20 answered · 0 correct

06

Frequently Asked Questions

10 Questions

What does HSG.CO.A.3 mean?

HSG.CO.A.3 means students can list and describe every rotation and reflection that maps a rectangle, parallelogram, trapezoid or regular polygon exactly onto itself. A complete answer names the center and angle of each rotation and the exact position of each line of reflection.

What does "carry a figure onto itself" mean?

It means that after the motion, the figure covers exactly the same points it covered before. The vertices may trade places, but the outline is unchanged. The 360° rotation always does this, so it is usually mentioned but not counted as an interesting symmetry.

Why is the diagonal of a rectangle not a line of symmetry?

Because a diagonal of a rectangle that is not a square does not cut the corner angles in half. Reflecting across it therefore sends the other two corners to points outside the rectangle, and the image does not fit the original. Folding a paper rectangle along its diagonal shows this quickly: the flaps do not line up. Among rectangles, only a square, whose sides are all equal, has its diagonals as lines of reflection.

Does a parallelogram have line symmetry?

Not in general: a parallelogram with no right angles and unequal adjacent sides has only a 180° rotation about the intersection of its diagonals. Special parallelograms do have lines of reflection: a rectangle has two, a rhombus has two (its diagonals), and a square has four.

What symmetries does a trapezoid have?

A trapezoid that is not isosceles has none except the 360° rotation. An isosceles trapezoid has exactly one line of reflection, through the midpoints of the two bases. Under the inclusive definition (at least one pair of parallel sides), parallelograms also count as trapezoids and have their own symmetries, so it helps to say which definition a problem uses.

How many lines of symmetry does a regular polygon have?

A regular polygon with n sides has n lines of reflection and is carried onto itself by rotations of 360°/n and its multiples. For even n, half the lines pass through opposite vertices and half through midpoints of opposite sides. For odd n, each line passes through a vertex and the midpoint of the opposite side.

Is HSG.CO.A.3 the same as the grade 4 symmetry standard?

No, it goes further. In grade 4 (4.G.A.3) students recognize and draw lines of symmetry by folding. HSG.CO.A.3 adds rotations, asks for a complete and precise list of motions for specific families of polygons, and treats those motions as transformations of the plane.

What is a common mistake on symmetry questions?

Giving an incomplete list. Students often find the lines of reflection but forget rotations, or list only the smallest rotation of a regular polygon instead of all its multiples below 360°. Another frequent error is using the wrong center, such as a vertex instead of the intersection of the diagonals.

How can students check their answers?

Use tracing paper or coordinates. Trace the figure, apply the motion to the tracing, and check that it fits exactly. With coordinates, apply the rule to every vertex and check that the set of image vertices is the same as the original set.

Where do these symmetries show up later?

They support proofs about parallelograms in HSG.CO.C.11, where a 180° rotation explains why opposite sides and angles are congruent, and constructions of regular polygons inscribed in circles in HSG.CO.D.13. They also appear in design, from floor tiles to logos.