If we break up the word 'equilateral', it becomes equal-lateral (‘lateral’ means ‘side’), so it’s easy to remember equilateral triangles have all equal sides.
Matching dashes through two or more sides or angles shows that they are equal.
Isosceles triangles Discuss and discover
Investigate a triangle with two equal sides. Work with a classmate to draw a triangle where the two sides of the triangle are both 7 cm and the third side is 6 cm.
(i) Using a protractor, fi nd the measure of each of the interior angles.
(ii) Repeat this activity for two more triangles, where only two sides are equal in length.
(iii) What have you discovered about the angles opposite the equal sides?
An isosceles triangle has two equal sides and two equal angles.
The word isosceles is Greek for ‘equal legs’. We have two legs, so it’s easy to remember isosceles triangles have two equal sides and two equal angles.
Examples of isosceles triangles (diagrams not to scale): 5.7 cm
7 cm 50° 9 cm Scalene triangles Discuss and discover
Investigate a triangle with no equal sides. Work with a classmate to draw a triangle where the sides of the triangle are 8 cm, 7 cm and 6 cm.
(i) Using a protractor, fi nd the measure of each of three interior angles.
(ii) Repeat this activity for two more triangles, where none of the sides of the sides are equal in length.
(iii) What have you discovered about the angles? 80° 7 cm 50° 66° 66° 7 cm 48° 7 cm 4 cm 74° 74° 7.2 cm 32° 7.2 cm