PR ll AD
PR ll BC => PR ll AD ll BC ;
PD = PC => P mijl. [CD]
R ∈ AB , R mijl. [AB]
=> PD = PC = BC = AD = AB / 2 ;
AB = DC ;
=> ADPR si RPCB - 2 romburi congruente => A ABCD = 2A romb
= 2A ADPR = 2 × AP × DR / 2 = AP × DR , AP × DR = AP × BP ;
=> A ABCD = AP × BP ;