通过讲究个人饮食卫生能够预防的传染病是（　　）

如图（1），将线段AB绕点A逆时针旋转2α（0°＜α＜90°）至AC，P是过A，B，C的三点圆上任意一点．

（1）当α=30°时，如图（1），求证：PC=PA+PB；

（2）当α=45°时，如图（2），PA，PB，PC三条线段间是否还具有上述数量关系？若有，请说明理由；若不具有，请探索它们的数量关系．

用适当的介词填空

①As a student, you should focus ____ your school work rather than some other things.

②Working in so much noise can cause damage ____ your hearing.

③His laziness and carelessness led ____ his failure in the examination.

④I am still a little confused____ what happened.

⑤I don’t want to make comments  ____ other people’s privacy.