Let us consider a right angled triangle.
In the triange, angle CAB is 90.
The base is AB , the height or altitude is, AC and the third side BC is called hypotenuse.
According to Pythagoras theorem ,if the length of two sides are given , the third side can be calculated as,

This is Pythagoras theorem. So in the drwan triangle we have,
![\begin{gathered} BC^2=AB^2+AC^2 \\ BC=\sqrt[]{AB^2+AC^2} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/iixjiyen71aeqz189gibf9v25n6rsicmvv.png)