Math lesson plan · 10th grade (Year 11 in England) · 45 minutes
Circle theorems: cyclic quadrilaterals and angles in the same segment
Geometry
CCSS HSG.C.A.3England: GCSE Higher Geometry
Objective and success criteria
Students will: Circle theorems: cyclic quadrilaterals and angles in the same segment.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Circle theorems
Leads to: Name the circle theorem that justifies a step
Materials: Rulers and protractors, Squared or dotted paper, Shape cut-outs, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Circle theorems). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- A triangle is drawn in a semicircle with one side the diameter. One of the other angles is 78°. What is the third angle (not the right angle)? → 12Prompt: The angle in a semicircle is 90°.
- Two angles at the circumference stand on the same arc. One is 37°. What is the other? → 37Prompt: Angles in the same segment are equal.
- The angle at the circumference is 77°, standing on the same arc as the angle at the centre. What is the angle at the centre? → 154Prompt: The angle at the centre is twice the angle at the circumference.
Hook · 5 min
- 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: Circle theorems: cyclic quadrilaterals and angles in the same segment. Read the success criteria together.
I do: model it · 10 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: ABCD is a cyclic quadrilateral. ∠DAB = 128°. Find ∠BCD.
- Step 1: 180° − 128° = 52°
- Answer: 52
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- ABCD is a cyclic quadrilateral. ∠ABC = 117°. Find ∠ADC. → 63Prompt: Are the two angles opposite each other?
- A, B, C and D lie on a circle, and the chords AC and BD cross. ∠ABD = 38° and ∠BAC = 49°. Find ∠ACD. → 38Prompt: ∠ABD and ∠ACD both stand on the chord AD.
You do: independent practice · 10 min
- 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.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- A, B, C and D lie on a circle, and the chords AC and BD cross. ∠ABD = 54° and ∠BAC = 55°. Find ∠ACD. → 54Prompt: ∠ABD and ∠ACD both stand on the chord AD.
- ABCD is a cyclic quadrilateral. ∠DAB = 102°. Find ∠BCD. → 78Prompt: Are the two angles opposite each other?
Watch out for
- Thinking a square turned on its corner is no longer a square.
- Reading the wrong scale on a protractor.
Support and stretch
Support
- Review first: Circle theorems.
- Use concrete materials: rulers and protractors.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Move on to: Name the circle theorem that justifies a step.
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/math/grade-10.
Free lesson plan from eurekami.com/lesson-plan-builder