Beyond the Basic Formulas —  Triangles Revisited:  Heron's Formula  If the height is unknown, but all three sides a, b, and c are known — then the Area of the triangle can be found by using the formula...  where s represents the semi-Perimeter,  i.e.,  s  =  P ÷ 2 Pythagorean Theorem In a "right triangle" where c represents the length of the hypotenuse (i.e., the longest side opposite of the 90° angle), the three sides a, b, and c are related according to the formula... a2 + b2 = c2