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How I wish I'd taught... shape [The Triangle]

Triangle: literally means three (tri) angles and so has 3 interior angles. But triangles are very special due to the wide variety of types of their properties.

Triangles have interior angles which sum to 180°. But why?

Acute triangles

An acute triangle has interior angles that are all acute (<90°).

Right angle

A right angle triangle has one angle equal to 90°

Obtuse triangles

An obtuse triangle has one obtuse angle.

Equilateral triangles

An equilateral triangle has three sides of equal length, and therefore all three interior angles are equal 60°.

Isosceles triangle

The word isosceles literally translates as 'equal legs'. An isosceles triangle is a triangle with at least two equal side lengths and to equal angles.

Isosceles right angle

Like an isosceles triangle but with a right angle. Two sides remain the same length and two angles are equivalent.


A scalene triangle has unequal sides and angles.

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