# Finding unknown lengths

**Pythagoras’ Theorem**

The Pythagoras theorem is only applicable to right angled triangles but can be used to find the length of a missing side if the side of the other two lengths is given.

For example, If *a* = 5cm and *b* = 12cm what is *c*?

*a*² + *b*² = *c*²

5² + 12² = *c*²

*c*² = 169

c = √169

c = 13 cm