Constructing triangles

Triangles and other shapes can be accurately constructed using a protractor, a pair of compasses and a ruler.

curriculum-key-fact
In order to draw a triangle, three properties are needed:
  • SAS - Side Angle Side
  • ASA - Angle Side Angle
  • SSS - Side Side Side

Constructing triangles given SAS

The lengths of two sides and the size of the angle in between are needed.

Example

Make an accurate drawing of a triangle with lengths 4 cm and 6 cm with an angle of 40° in between.

A horizontal line labelled 6 cm

Draw the longest side using a ruler

Constructing triangles given ASA

The sizes of two angles and the length of the side in between are needed.

Example

Make an accurate drawing of a triangle with the angles 40° and 70°, with a side of length 8 cm in between.

A horizontal line labelled 6 cm

Draw the side using a ruler

Constructing triangles given SSS

The lengths of all three sides are needed.

Example

Make an accurate drawing of a triangle with sides of length 3 cm, 5 cm and 6 cm.

A horizontal line labelled 6 cm

Draw the longest side using a ruler

As with all constructions, ensure that the construction lines are not rubbed out. These show that the triangles were accurately constructed.