共50条信息
如图,在一个\({{60}^{o}}\)的二面角的棱上有两点\(A,B\),线段\(AC,BD\)分别在这两个面内,且都垂直于棱\(AB\),\(AB=AC=a\),\(BD=2a\),则\(CD\)的长为\((\) \()\)
二面角\(α-\) \(l\)\(-β\)等于\(120^{\circ}\),\(A\)、\(B\)是棱 \(l\)上两点,\(AC\)、\(BD\)分别在半平面\(α\)、\(β\)内,\(AC⊥\) \(l\),\(BD⊥\) \(l\),且\(AB=AC=BD=1\),则\(CD\)的长等于( )
如图,菱形\(ABCD\)的对角线\(AC\)与\(BD\)交于点\(O\),\(AB\)\(=5\),\(AC\)\(=6\),点\(E\),\(F\)分别在\(AD\),\(CD\)上,\(AE\)\(=\)\(CF\)\(=\dfrac{5}{4}\),\(EF\)交\(BD\)于点\(H\)\(.\)将\(\triangle \)\(DEF\)沿\(EF\)折到\(\triangle {D}{{'}}EF\)的位置,\(O{D}{{'}}=\sqrt{10}\) .
\((\)Ⅰ\()\)证明:\({D}{{'}}H\bot \)平面\(ABCD\);
\((\)Ⅱ\()\)求二面角\(B-{D}{{'}}A-C\)的正弦值\(.\)
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