如图,在四棱锥中,平面平面,,. (1)证明:平面; (2)求二面角的大小 答案:(I)在直角梯形中,由,得,,由,则,即,又平面平面,从而平面,所以,又,从而平面; (II)方法一:作,与交于点,过点作,与交于点,连结,由(I)知,,则,,所以是二面角的平面角,在直角梯形中,由,得,又平面平面,得平面,从而,,由于平面,得:,在中,由,,得, 在中,, If you leave the club, you will not be
________back in.
A. remembered B.
moved C. turned D.
admitted