• Quick retrieval from earlier learning (Dilations (enlargements) from the origin). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Dilate (4, 5) by a scale factor of 4 with centre the origin. Type the image like (2, −3). → (16, 20)Prompt: Multiply both coordinates by the scale factor.
2. A(−12, 8) is dilated about the origin to A′(−30, 20). What is the scale factor? Give a fraction or decimal. → 5/2 (2 1/2)Prompt: Divide an image coordinate by the matching original coordinate.
3. A 6 by 5 rectangle is dilated by scale factor 4. What is the perimeter of the image? → 88Prompt: Every length is multiplied by the scale factor.

• Show a photo of a building, a tile pattern or a road sign and ask what shapes and angles students can spot.
• Share the objective: Find missing sides in similar shapes. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Two triangles are similar. The small one has sides 7 cm and 8 cm. The matching side to 7 cm on the large triangle is 14 cm. How long is the side matching 8 cm?
• Step 1: k = 2
• Step 2: 8 × 2 = 16 cm
• Answer: 16
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Two triangles are similar. The small one has sides 7 cm and 3 cm. The matching side to 7 cm on the large triangle is 28 cm. How long is the side matching 3 cm? → 12Prompt: Find the scale factor from the matching pair.
2. Grace is 3.5 m tall and casts a 2 m shadow. At the same moment a tree casts a 7 m shadow. How tall is the tree, in m? Round to 1 decimal place if needed. → 12.3Prompt: The sun's rays make similar triangles.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Arjun is 2 m tall and casts a 2.5 m shadow. At the same moment a tree casts a 19 m shadow. How tall is the tree, in m? Round to 1 decimal place if needed. → 15.2Prompt: The sun's rays make similar triangles.
2. Two triangles are similar. The small one has sides 3 cm and 4 cm. The matching side to 3 cm on the large triangle is 9 cm. How long is the side matching 4 cm? → 12Prompt: Find the scale factor from the matching pair.

Free online practice with hints and printable worksheets for this skill: eurekami.com/math/grade-8.