From Förberedande kurs i matematik 1

A right-angled triangle is a triangle in which one of the angles is \displaystyle 90^{\circ }. The side which is opposite the \displaystyle 90^{\circ } -angle is called the hypotenuse (marked \displaystyle x in the triangle) and the others are called opposite and the adjacent.

With the help of Pythagoras' theorem, we can write a relation between the sides of a right angled triangle:

\displaystyle x^{2}=30^{2}+40^{2}

This equation gives us that

\displaystyle \begin{align} & x=\sqrt{30^{2}+40^{2}}=\sqrt{900+1600}=\sqrt{2500} \\ & =\sqrt{25\centerdot 100}=\sqrt{5^{2}\centerdot 10^{2}}=5\centerdot 10=50 \\ \end{align}