题目

Happiness is for everyone．You don’t need to care about those who have beautiful houses w【1】 large gardens and swimming pools．You don’t need to care about those w【2】 have nice cars and a lot of money and so on．Why? Because those who have big houses may often feel l【3】 and those who have cars may want to w【4】 on the country roads in their free time．In fact, happiness is always around you if you put your heart into it．When you have d【5】 learning your lessons, your friends will help you; When you study, your parents are always l【6】 after your life and your hea一个多面体的直观图及三视图如图所示：(其中M、N别是AF、BC的中点)．  (I)求证：MN//平面CDEF； (Ⅱ)求二面角D-MN-B的余弦值绝对值．     三视图