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).
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.
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