ἀγεωμέτρητοσ μηδεὶσ εἰσίτω

For a right-angled triangle, the square of the length of the hypotenuse ($c$) is equal to the sum of the squares of the lengths of the other two sides ($a$ and $b$). This can be represented as: \[ c^2 = a^2 + b^2 \]