$\begin{array}{l}
 {P_{trougla}} = \frac{1}{2}a(r - h) \\ 
 {\left( {\frac{a}{2}} \right)^2} = {r^2} - {(r - h)^2} = 2rh - {h^2} \\ 
 {a^2} = 4(2rh - {h^2}) \\ 
 a = 2\sqrt {2rh - {h^2}}  \\ 
 {P_{trougla}} = (r - h)\sqrt {2rh - {h^2}}  \\ 
 \end{array}$