填空：。如图，∠1=100°，∠2=100°，∠3=120°，求∠4的度数

解：已知,∠1=∠2=100°
根据          ______                                                  
∴m∥n
又根据          ______                       
∴∠     =∠     
∵∠3=120°        ∴∠4=120°