|a|=√[x1^2+y1^2]

|b|=√[x2^2+y2^2]

a*b=(x1,y1)(x2,y2)=x1x2+y1y2

cos=a*b/[|a|*|b|] =(x1x2+y1y2)/[√[x1^2+y1^2]*√[x2^2+y2^2]]