TeX Quellcode:

\begin{array}{rcl} \dfrac{f}{2} &=& \sqrt{a^2-\left(\dfrac{e}{2}\right)^2} \cr \dfrac{f}{2} &=& \sqrt{(8 \, cm)^2-\left(\dfrac{13 \, cm}{2}\right)^2}\cr \dfrac{f}{2} &=& \dfrac{\sqrt{87}}{2}\, cm \end{array}