1
AC//A1C1
A1C1不在面ACD内
A1C1//平面ACD
2
A1D=√2a AD=√2a
A1D^2+AD^2=AA1^2
A1D⊥AD
AC⊥AA1
AC⊥AB
AC⊥平面AA1BB1
A1D⊥AC
A1D⊥平面ADC