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解法一：                                   ，　           

依题设知，．

（Ⅰ）连结交于点，则．

由三垂线定理知，．······························································· 3分

在平面内，连结交于点，

由于，

故，，

与互余．

于是．

与平面内两条相交直线都垂直，

所以平面．········································································· 6分

（Ⅱ）作，垂足为，连结．由三垂线定理知，

故是二面角的平面角．··············································· 8分

，

，．

，．

又，．

．

所以二面角的大小为．··············· 12分

解法二：

以为坐标原点，射线为轴的正半轴，

建立如图所示直角坐标系．

依题设，．

，

．······························································· 3分

（Ⅰ）因为，，

故，．

又，

所以平面．········································································· 6分

（Ⅱ）设向量是平面的法向量，则

，．

故，．

令，则，，．·············································· 9分

等于二面角的平面角，

．

所以二面角的大小为．

同答案