如图1，在平面直角坐标系xOy中，点A的坐标为（0，1），取一点B（b，0），连接AB，做线段AB的垂直平分线l₁，过点B作x轴的垂线l₂，记l₁，l₂的交点为P．

    It is Sunday morning，Helen and her brother David are 1a walk in the park. There are lots of signs in the park. Helen knows a lot 2 them. But David 3. Suddenly Helen sees a boy climbing a tree. She says to the boy,"Don't climb the tree. Can't you see the 4Over there?" The boy jumps off the tree soon. David points to the sign and says to Helen，"So the sign means we 5climb trees，right?"  "Yes." says Helen.