Hello, I'm Helen. I'm twelve years old. I'm in Garden Primary School. Jane is my classmate. We're good friends. I've got a green bicycle. It's new. I often ride the bicycle to go out and play. Today is Sunday. Jane and I go to visit Aunt Wang by bicycle. She lives in Pudong. She works in a hospital. She likes to make sick people better. She works very hard. It's 5: 45 now. It's dark outside. We must say goodbye to Aunt Wang.

如图1，在等腰Rt△ABC中，D为斜边AC边上一点，以CD为直角边，点C为直角顶点，向外构造等腰Rt△CDE．动点P从点A出发，以1个单位/s的速度，沿着折线A﹣D﹣E运动．在运动过程中，△BCP的面积S与运动时间t（s）的函数图象如图2所示，则BC的长是____