Planar vector angle online calculator
\(|a|=√[x1^2+y1^2]\)
\(|b|=√[x2^2+y2^2]\)
\(a \times b=(x1,y1)(x2,y2)=x1x2+y1y2\)
\(cos=a \times b/[|a| \times |b|] =(x1x2+y1y2)/[√[x1^2+y1^2] \times √[x2^2+y2^2]]\)
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