题目

In our daily lives, Wechat (微信) is really hot now. Many people express themselves, exchange ideas and deliver information by Wechat.There are a number of reasons for Wechat to be popular. To begin with, Wechat is a cheap way of communication, which cuts down a great deal of the cost made by phone calls. Next, Wechat is to the taste of the most of people. Its attractive and lots of functions(功能) are loved wildly. What `s more, convenience also leads to its popularity. It is available no matter where we are. However, it has problems, too. First of all, our IDs might be revealed when we u如图，在平面直角坐标系xOy中，抛物线y=ax2+bx+c经过A、B、C三点，已知点A（-3，0），B（0，3），C（1，0）．（1）求此抛物线的解析式．（2）点P是直线AB上方的抛物线上一动点，（不与点A、B重合），过点P作x轴的垂线，垂足为F，交直线AB于点E，作PD⊥AB于点D．①动点P在什么位置时，△PDE的周长最大，求出此时P点的坐标；②连接PA，以AP为边作图示一侧的正方形APMN，随着点P的运动，正方形的大小、位置也随之改变．当顶点M或N恰好落在抛物线对称轴上时，求出对应的P点的坐标．（结果保留根号）