$d^2=(12\mathrm{c}\mathrm{m}{)}^2+(5\mathrm{c}\mathrm{m}{)}^{2}d^2=(12\backslash ,\backslash mathrm\{cm\})^2+(5\backslash ,\backslash mathrm\{cm\})^2$

$d^2=144{\mathrm{c}\mathrm{m}}^2+25{\mathrm{c}\mathrm{m}}^{2}d^2=144\backslash ,\backslash mathrm\{cm\}^2+25\backslash ,\backslash mathrm\{cm\}^2$

$d^2=169{\mathrm{c}\mathrm{m}}^{2}d^2=169\backslash ,\backslash mathrm\{cm\}^2$

Ziehe die Wurzel aus 169

$d=13\mathrm{c}\mathrm{m}d=13\backslash ,\backslash mathrm\{cm\}$

Die Diagonale $dd$ ist $13\mathrm{c}\mathrm{m}13\backslash ,\backslash mathrm\{cm\}$.