Mathematics

Advanced

50 mins

Teacher/Student led

Learning Goals

Learning Outcomes

Lesson Files

+80 XP

Teacher-led from the board. Pupils do not need a device, but can follow on one.

What you need:
IWB/Projector/Large Screen
Compass
Protractor

Shape and Angle Construction Review

Review construction and angle skills by building a triangle from three given sides, measuring two angles, then reasoning the third using the 180° rule. Explore when measuring fails and reasoning succeeds.

Warning: Your screen size is quite small for viewing this. Rotate your device into landscape mode if you can.

1 Getting Started
2 Watch and Notice
3 Try It Together
4 Build It in Your Copy
5 Class Challenge
6 What Did We Notice?
7 What's Next
Pupil practice

Your turn

Your teacher is running this lesson on the board. Here is the part you can try yourself.

Your written work is on page 81

Circle tool

1 - Getting Started ~4 mins

Illustration for Getting Started Imagine a triangle with one side already ruled and one angle already marked, but the last corner not yet joined.

To finish that triangle, some parts you would construct (draw carefully with your compass and ruler), and some parts you would work out (figure out with maths). Which is which?

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~9 mins

Look at the triangle on the board, reconstructed from three given sides (6 cm, 5 cm, 4 cm) using compass arcs. The two arcs cross to fix the last corner, so only one triangle can close.

Now watch what happens with the angles. Once we know two of the triangle's angles, we do not need a protractor for the third. All three angles add to 180°, so the third one is 180° take away the other two. That is an angle we work out, not one we measure.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~11 mins

Key point

Now we work through one full review task together: construct a triangle from its three sides (7 cm, 6 cm, 5 cm), then find its angles. We will measure two of them with the protractor. For the third, we do not measure at all: we take the two we found away from 180° (180° − first angle − second angle), then check it with the protractor to see it matches.

We build in this order: rule the 7 cm base, swing a 6 cm arc from one end, swing a 5 cm arc from the other, join the crossing point, then measure and reason the angles.

Construct the 7-6-5 triangle, then find its angles

+15 XP

4 - Build It in Your Copy ~3 mins

COPYBOOK MOMENT

In your maths copy, complete the review task with your own ruler, compass and protractor: construct the triangle from the three sides (7 cm, 6 cm, 5 cm), measure two angles, then work out the third by taking those two away from 180°. Check that all three angles add to 180°.

Ring the angle you worked out rather than measured.

+10 XP

Pupil practice

Module 7 · 2D Shape, 3D Shape and Angles Shape & Space

Lesson 87 · Shape and Angle Construction Review

Download Activity Book page (PDF)

123learn · Online learning platform

Unlock the full learning experience

You're previewing this lesson. Get full access to this lesson and hundreds more — each one ready to teach, with interactive activities, printable resources and pupil progress tracking built in.

Hundreds of curriculum-aligned lessons

Interactive activities in every lesson

Printable resources & progress tracking

Get started

Copyright Notice
This lesson is copyright of 123Learn.ie 2017 - 2026. Unauthorised use, copying or distribution is not allowed.