题目

单词拼写:根据首字母或汉语注释写出单词的正确形式。  1．His health s   ________   terribly from heavy drinking in the past．  2．As far as I am c  ______  ，I can't agree with what you have said．  3．The worker i  _____  his own danger and jumped into the river to save the boy．  4．Now t  ____   like to surf the internet；some even forget his own studies． 5．Soon the robber was found   ________ (躲藏) in a mountain village． 6．I am now in a difficult   _______ (情形，情况)．I badly need your help．  7．The news quite   ____5．如图甲，在边长为4的等边△ABC中，点E，F分别为AB，AC上一点，且EF∥BC，EF=2a，沿EF将△AEF折起，使得平面AEF⊥平面EFCB，形成一个如图乙所示的四棱锥，设O为EF的中点．（1）求证：AO⊥BE；（2）当a为何值时，四棱锥A-EFCB的体积最大，并求出最大值．