[法一]取CC'的中点为H,由EF//GH,EFGH共面取A'D'中点为I,C'D'中点为J,由GH//IJ,GHJI共面.由EG//HJ,EFGHJI共面该平面平行于三角形AD'C.BB'垂直于ABCD,则AC垂直于BB'.又由AC垂直于BD,所以AC垂直于BB'D'D,BD'垂直于AC.同...