[math]AB\parallel DC\wedge AD\parallel BC\wedge\angle A_1=\angle A_2\wedge\angle B_1=\angle B_2\wedge\angle C_1=\angle C_2\wedge\angle D_1=\angle D_2[/math][br] [math]\Downarrow[/math][br][math]AB=BC=CD=DA[/math]