21题,21题,
21题,(1)∵AB∥DF,∴∠D=∠ABC,∴cos∠D=cos∠ABC= AB BC = 4 4.25 ≈0.94,∴∠D≈20°;(2)∵sinD= EF DE ,∴EF=DEsin∠D=85sin20°≈85×0.34=28.9( 共需台阶28.9×100÷17=170级.