题目

-What’s your problem，Madam? -I'm a teenager’s mother．I can’t get along well with my daughter．Sometimes I don’t know________． -Love and understanding． A．what does my daughter want to get B．if my daughter wants to get love and understanding（理解） C．what my daughter needs most D．why my daughter gets annoyed very often 答案：C9．在四棱锥P-ABCD中，设底面ABCD是边长为1的正方形，PA⊥面ABCD．（1）求证：PC⊥BD；（2）过BD且与直线PC垂直的平面与PC交于点E，当三棱锥E-BCD的体积最大时，求二面角E-BD-C的大小．