Pythagoras' theorem is a formula you can use to calculate the length of any of the sides on a right-angled triangle or the distance between two points.

Part of

Pythagoras' theorem states that for all right-angled triangles, **'The square on the** hypotenuse **is equal to the sum of the squares on the other two sides'**. The hypotenuse is the longest side and it's always opposite the right angle.

In this triangle and angle is a right angle.

Pythagoras' theorem only works for right-angled triangles, so you can use it to test whether a triangle has a right angle or not.

In the triangle above, if the angle is acute.

In the triangle above, if the angle is obtuse.

- Question
Which of the following triangles is right-angled?

**a)****b)****c)****a)**is greater than ,

so angle must be

**obtuse**.**b)**is equal to

so angle must be a

**right angle**.**c)**is less than

so angle must be

**acute**.So, it is triangle

**b**which is right-angled.