2.1 Triangles

Triangles are the most basic of the polygons (a polygon is a straight-sided shape). They have three sides, the smallest number of sides it is possible to have. Triangles also make a rigid structure, as any engineer will know.

You will know the names of the triangles which have further properties in addition to having three sides (Figure 6).

Figure 6 Types of triangle

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A table showing diagrams of a right-angled, triangle, an isosceles triangle and an equilateral triangle with their labels in one column and their properties in another column.

Figure 6 Types of triangle

In addition, the isosceles right-angled triangle has a right angle, two equal sides, and two equal angles of 45⁰ each.

Acute-angled triangles have three angles all smaller than a right angle.

Obtuse-angled triangles have one angle that is bigger than a right angle.