a) comparam ΔAOB si ΔBOC
avem OB-latura comuna
<AOB≡<BOC
AO≡CO
=> (L.U.L) ΔAOB≡ΔBOC => AB ≡ BC => ΔABC-isoscel
b)la fel se demonstreaza ca ΔBCD-isoscel => BC≡CD
se compara ΔAOC si ΔBOD => AC≡BD
compara ΔABC si ΔBCD
AB≡BC
BC≡CD
AC≡BD
=> (L.L.L) ΔABC≡ΔBCD
c) se compara ΔAOD si ΔBOE
AO≡OB
OD≡OE
<AOD≡<BOE(formate din 3 unghiuri congr)
=> (L.U.L) ΔAOD≡ΔBOE => AD≡BE