pAIrmatic Lessons

Rotations

8.G.1Grade 8Geometry20 practice questions

A rotation turns a figure around a fixed point, like a wheel turning on its hub. Every point stays the same distance from the center.

−1−111223344(3, 1)

Know these first

What you'll be able to do

The big idea

A rotation needs three things: a center, an angle and a direction. Counterclockwise is the standard direction unless you're told otherwise.

About the origin, the rules are short: 90° counterclockwise (x, y) → (−y, x); 180° (x, y) → (−x, −y); 270° counterclockwise (x, y) → (y, −x). A 90° clockwise turn is the same as 270° counterclockwise.

To rotate about another point, slide everything so that point sits on the origin, rotate, then slide back. Rotations keep lengths, angles and orientation.

Words you need

Center of rotation
The fixed point everything turns around.
Counterclockwise
The opposite way to a clock's hands - the standard direction.
Quarter-turn
A 90° rotation. Four quarter-turns bring a figure back to the start.

Rotation rules about the origin

TurnRuleExample
90° counterclockwise(x, y) → (−y, x)(3, 1) → (−1, 3)
180°(x, y) → (−x, −y)(3, 1) → (−3, −1)
270° counterclockwise = 90° clockwise(x, y) → (y, −x)(3, 1) → (1, −3)

Worked examples

Example 1

Rotate the point (3, 1) 90° counterclockwise about the origin.

  1. Use (x, y) → (−y, x).
  2. −y = −1 and x = 3, so the image is (−1, 3).

Answer: (−1, 3)

−1−111223344(3, 1)

Example 2

Rotate the point (2, −4) 180° about the origin.

  1. A half-turn changes the sign of both coordinates: (x, y) → (−x, −y).
  2. (2, −4) → (−2, 4).

Answer: (−2, 4)

−5−5−4−4−3−3−2−2−1−1112233(2, −4)

Example 3

Rotate triangle ABC with A(1, 1), B(4, 1) and C(1, 3) 90° clockwise about the origin.

  1. 90° clockwise is the same as 270° counterclockwise: (x, y) → (y, −x).
  2. A(1, 1) → (1, −1), B(4, 1) → (1, −4), C(1, 3) → (3, −1).

Answer: (1, −1), (1, −4), (3, −1)

−1−11122334455ABC

Common mistakes

Mixing up the 90° rules. Counterclockwise is (−y, x). Check with an easy point: (1, 0) should turn to (0, 1), straight up.

Forgetting which way is clockwise. Clockwise follows a clock's hands: from 12 to 3. 90° clockwise = 270° counterclockwise.

Using the origin when the center is elsewhere. Shift so the center is at the origin, rotate, then shift back.

Practice: 20 questions

12 of 20 are word problems. For a word problem, sketch it first and mark what you know.

Tier 1 Foundations Use the rule directly

  1. Rotate the point (4, 2) 180° about the origin.
    −1−11122334455(4, 2)
  2. Rotate the point (1, 5) 90° counterclockwise about the origin.
    −1−1112233445566(1, 5)
  3. A windmill blade's tip is at (0, 6). It turns 90° counterclockwise about the hub at the origin. Where is the tip now? Word problem
  4. The tip of a clock hand is at (0, 4). It turns 90° clockwise about the center at the origin. Where is the tip now? Word problem
  5. A spinner's pointer tip at (3, 0) rotates 180° about the origin. Where does it point now? Word problem

Tier 2 Intermediate One more step to think through

  1. Rotate the point (2, −3) 270° counterclockwise about the origin.
    −4−4−3−3−2−2−1−1112233(2, −3)
  2. Rotate triangle ABC with A(1, 2), B(3, 2) and C(1, 5) 90° counterclockwise about the origin.
    −1−1112233445566ABC
  3. Point (2, 5) maps to (−2, −5), and point (−1, 3) maps to (1, −3). Which rotation about the origin is this?
    −6−6−5−5−4−4−3−3−2−2−1−1112233445566ABA′B′
  4. A Ferris wheel seat at (5, 0) turns 90° clockwise about the center at the origin. Where is the seat now? Word problem
  5. A game sprite has corners at (1, 1) and (2, 3). It rotates 90° counterclockwise about the origin. Where are the corners now? Word problem
  6. A marker at (−4, 1) on a turntable rotates 90° clockwise about the origin. Where is it now? Word problem
  7. A segment from (0, 0) to (5, 12) is rotated 90° about the origin. How long is the image? Word problem

Tier 3 Advanced Several steps - find the hidden structure

  1. Rotate the point (6, 2) 90° counterclockwise about the point (2, 2).
    −1−111223344556677(6, 2)orange dot = center (2, 2)
  2. The point (3, 1) is rotated 90° counterclockwise about the origin, then 180° about the origin. Where does it end up?
    −1−111223344(3, 1)
  3. A door's hinge is at (1, 1), and its handle is at (4, 1). The door swings 90° counterclockwise about the hinge. Where is the handle now? Word problem
  4. A tile's corner moves from (2, 3) to (−3, 2) when the tile turns about the origin. Which rotation is this? Word problem
  5. A satellite dish marker at (−2, 5) rotates 270° counterclockwise about the origin. Where is it now? Word problem

Tier 4 Challenge Stretch problems, some above grade level

  1. Rotate the point (5, 3) 90° clockwise about the point (1, 1). Above grade level
    −1−1112233445566(5, 3)orange dot = center (1, 1)
  2. A robot arm's tip at (3, 4) rotates 90° counterclockwise about the origin and is then reflected over the x-axis. Where does it end up? Word problem
  3. A figure is rotated 90° clockwise three times. What single rotation puts it in the same place? Word problem

Answers

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