证明：∵∠A+∠C=90°+90°=180°∴∠ABC+∠ADC=180°（四边形内角和180°）∵BE平分∠ABC∴∠CBE=1/2∠ABC∵DF平分∠ADC∴∠CDF=1/2∠ADC ∴∠CBE +∠CDF=1/2∠ABC+1/2∠ADC =90°∵∠CBE+∠CEB=90°∴∠CDF=∠CEB∴BE//DF
