Ue use the theorem:
The equation relating the lengths of the sides a, b and c, often called the "Pythagorean equation": \(a^{2} + b^{2} = c^{2} ;\)
where c represents the length of the hypotenuse (the side opposite the right angle) and a and b the lengths of the triangle's other two sides.
\(c^{2} = 20^{2} + 21^{2}\);
\(c^{2} = 841 ;
c = \sqrt{841} ;
c = 29 cm.\)
