mx = (4+2)/2 = 3
my = (1+5)/2 = 3
M点坐标为(3,3)
向量om的长度为根号下(3*3+3*3) = 3×根号2
N的坐标用M,A坐标平均:((4+3)/2,(1+3)/2)
即(7/2,2)