AD||BC AB||DC
∵∠A=∠C,∠B=∠D
∴∠A+∠B+∠C+∠D=360°
∴2∠A+2∠B=360° 2∠C+2∠D=360°
∴∠A+∠B=180° ∠C+∠D=180°
∴AD‖BC AB‖DC