point1(x1,y1) A
point2(x2,y2) B
point3(x3,y3) C
cos(A) = (x2-x1)*(x3-x1) + (y2-y1)*(y3-y1) / sqrt([(x2-x1)*(x2-x1) + (y2-y1)*(y2-y1)]*[(x3-x1)*(x3-x1)+(y3-y1)*(y3-y1)]);
在图像处理中检测图形时很有用
