a) Daca ΔADB si ΔDBC isoscele si dreptunghice (pentru ca ∡DBC=90grade )⇒ AD=DB=BC = 4cm si
∡DAB≡∡ABD≡∡DCB≡∡CDB=45 grade
∡ADC=∡ADB+∡CDB= 90+45=135 grade
b) AD=BD=BC=4 cm
c) A ABCD= A ADB + A BDC
ΔBCD≡ΔDAB
⇒ A ABCD= 2 * A ADB= 2 * 4*4:2= 16 cm ²