共50条信息
已知直棱柱\({ABC}{-}A_{1}B_{1}C_{1}{,}{∠}{ACB}{=}60^{{∘}}{,}{AC}{=}{BC}{=}4{,}AA_{1}{=}6{,}E\)、\(F\)分别是棱\(CC_{1}\)、\(AB\)的中点.
如图,在多面体\(ABCDEF\)中,\(ABCD\)是正方形,\(BF⊥\)平面\(ABCD\),\(DE⊥\)平面\(ABCD\),\(BF=DE\),点\(M\)为棱\(AE\)的中点.
\((\)Ⅰ\()\)求证:平面\(BDM/\!/\)平面\(EFC\);
\((\)Ⅱ\()\)若\(AB=1\),\(BF=2\),求三棱锥\(A-CEF\)的体积.
如图所示,在直三棱柱\(ABC—A_{1}B_{1}C_{1}\)中,\(AC=3\),\(BC=4\),\(AB=5\),\(AA_{1}=4\),点\(D\)是\(AB\)的中点.
\((1)\)在棱\(A_{1}B_{1}\)上找一点\(D_{1}\),当\(D_{1}\)在何处时可使平面\(AC_{1}D_{1}/\!/\)平面\(CDB_{1}\),并证明你的结论;
\((2)\)求二面角\(B_{1}-CD-B\)大小的正切值.
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