设a的坐标(x1,y1)
b(x2,y2)
x1+x2=2
x1-x2=-6
x1=-2
x2=4
y1+y2=-8
y1-y2=-4
y1=-6
y2=-2
∴a=(-2,-6)
b=(4,-2)
cos=(a·b)/(|a|*|b|)=(-8+12)/(2√10*2√5)=√2/10