阅读理解

　　It's eight o'clock in the morning. The students are coming into the classroom. A girl is opening the windows. Some students are laughing and talking. Some others are listening to them. Some are reading books. Some are doing their homework.

　　Miss Yang is our English teacher, and she is standing in front of the blackboard and writing on it. Daming and Tony are wearing their new clothes today. Betty and Jim are cleaning their desks. They are all happy. Where are Lucy and Lily? Oh, they are playing chess on the floor.

如图，在平面直角坐标系中，二次函数y=ax²+c（a≠0）的图象过正方形ABOC的三个顶点A、B、C，则ac的值是____．