Triangles

Parts of a triangle: a triangle is a flat shape with three sides and three angles. The straight lines that make its sides meet at three points called vertices. Each vertex and the two sides beside it form one of the triangle's angles.

📖 In short

A triangle is a shape with three straight sides, three angles and three corners. It's the simplest polygon. We see it in a slice of pizza, a sloping house roof, the pyramids and road signs.

🤩 Did you know?

The angles of a triangle always add up to 180 degrees.
A triangle's area equals its base times its height, divided by 2.
A right triangle has a 90-degree angle.
A triangle is a kind of polygon.

🔎 Find out more

🌟 Adding up the angles

If you cut off the three corners of any paper triangle and put them side by side, they make a straight line! That's because the angles of any triangle always add up to 180 degrees.

🔍 Kinds by sides

An equilateral triangle has three equal sides, an isosceles triangle has two equal sides, and a scalene triangle has all sides different.

💡 Kinds by angles

A right triangle has a right angle like the corner of a book, an acute triangle has all small angles, and an obtuse triangle has one big angle bigger than a right angle.

🎯 The strongest shape

The triangle is a very strong shape that doesn't bend easily, so engineers use triangles to build bridges, towers and cranes, like the Eiffel Tower.

📸 More pictures

Kinds of triangles: triangles can be sorted by their angles and by how their sides compare. In the figure, blue marks show sides of equal length, and red marks show equal angles. The small gray box marks a right angle, that is 90°. A right triangle is one with a right angle (90°), and no triangle can have more than one right or obtuse angle.

🧩 Test yourself

1 What do the angles of a triangle add up to?

2 Why do engineers use triangles in bridges?

3 The angles of a triangle always add up to ?? degrees.

4 Which picture shows “Triangles”?