https://mooc1.chaoxing.com/mycourse/studentstudy?chapterId=732502874&courseId=232937933&clazzid=81765869&cpi=236968743&enc=cb93a8238917f1e8c7293e57817e5c93&mooc2=1&openc=69dcb6f5e81aa701212665f222f7b354 PPT中示例中求积相加再求平均值，最终得到的就是皮尔逊相关系数