Constructing Triangles with Compasses and Protractors

November 9, 2021

Differentiated Learning Objectives

All students should be able to construct a triangle using SSS.
Most students should be able to construct a triangle using SSS and SAS,
Some students should be able to construct a triangle using SSS, SAS and ASA.

Links to Lesson Resources (Members Only)

Starter/Introduction

The start of the lesson encourages students to work in pairs and discuss the symmetrical properties of a square. Encourage students to use mathematical keywords such as symmetry, reflection, square, rectangle, and triangle.

Constructing Triangles with Compasses and Protractors

Prompts / Questions to consider

What is reflective symmetry?
Which shape could be half a right-angled triangled triangle?

Constructing Triangles with Compasses and Protractors

Click here to view the video.

The key points when constructing triangles with compasses and protractors are:

Equipment Needed

Side, Angle, Side: Pair of compasses, rule and protractor

Angle, Side, Angle: Ruler and pair of compasses

Side, Side, Side: Pair of compasses and ruler

Right-angle, Hypotenuse, Side: Pair of compasses and ruler

Students need to observe their teacher using the same equipment they will use. If possible, the teacher should use a visualiser to work at their desk with the same equipment.

Encourage students to work to an accuracy of 2 mm and 2 degrees. Encourage students to work in pairs so they can check the accuracy of their peer’s work.

Prompts / Questions to consider

What type of triangle is it? (SAS, ASA, SSS, RHS)
What equipment is needed?
Which scale do I use on the protractor?

Plenary

The plenary challenges students to connect their understanding of the properties of quadrilaterals to constructions. This activity takes between 10 to 12 minutes.

Prompts / Questions to consider

How can I use the formula to work out the calculation?
Can I use a pair of brackets to make the calculation simpler?
How can I use the fraction and mixed number button on a calculator to check my solution?

Differentiation

Some students struggle with the dexterity of using a pair of compasses to a suitable degree of accuracy. Encourage students to rotate the paper/book rather than the compasses. Less able often use the incorrect scale on a 180Â° protraction. To support this, remind students about the difference between acute and obtuse angles.