teorema catetei
AB²=BDxBC ⇒BD =AB²/BC
AC²=DCxBC ⇒DC =AC²/BC
teorema inaltimii
AD²=BDxBC
BDxBC=AB²/BC xAC²/BC=(AB²xAC²)/BC² =(AB²xAC²)/(AB² +AC²)
1/BDxBC=1/[(AB²xAC²)/(AB² +AC²)]=(AB² +AC²)/(AB²xAC²)=AB²/(AB²xAC²)+
+AC²/(AB²xAC²)=1/AC²+1/AB²
1/AD²=1/BDxBC=1/AB²+1/AC²