证:因面ABC平行于面A1B1C1
故AD//A1D1
AD在平面ADC1中
故A1D1//面ADC1
A1B//面ADC1且A1D1与A1B相交于点A1
A1D1与A1B都在平面A1BD1中,
故面A1BD1平行于面AC1D