7. 如图,在直三棱柱ABC−A1B1C1ABC-A_{1}B_{1}C_{1}ABC−A1B1C1中,已知AB=BC=2AB=BC=2AB=BC=2,BB1=3BB_{1}=3BB1=3,AB⊥BCAB\bot BCAB⊥BC,DDD为ABABAB的中点.???(1)求异面直线BC1BC_{1}BC1与DCDCDC所成角的大小(用反三角函数表示);(2)求证:BC1//BC_{1}//BC1//平面A1CDA_{1}CDA1CD.???