The World Health Organization has given some advice on how to protect us from infected(感染) with H7N9.
If you want to learn English well, you should learn enough words by heart and do enough reading.
下列关系中，互相垂直的两条直线是（ ）
如图，已知⊙O的半径为5，锐角△ABC内接于⊙O，BD⊥AC于点D，AB=8，则tan∠CBD的值等于（ ）
