Right-Angled Triangle — free printable diagram

What is a right-angled triangle?

A right-angled triangle has one angle of exactly 90°, marked with a small square rather than an arc. Since the three angles add to 180°, the other two must add to 90°, so both are acute.

Sides

3

Equal sides

None required

Equal angles

None required

Angles add up to

180°

Lines of symmetry

0 (1 if isosceles)

Rotational symmetry

Order 1 (none)

• The side opposite the right angle is the hypotenuse, and it is always the longest side.
• Pythagoras' theorem applies only to right-angled triangles: a² + b² = c², where c is the hypotenuse.
• The two non-right angles are complementary — they add to 90° — so knowing one gives you the other immediately.
• Trigonometry (sine, cosine and tangent) is defined from the sides of a right-angled triangle, which is why this shape is introduced before any of it.
• A right-angled triangle can be scalene or isosceles, but never equilateral, because an equilateral triangle's angles are all 60°.