Dot Product and Distance (2D, 3D, ...)

a.k.a. Scalar Product

        / x1   / x2
 u.v =  | y1 . | y2  =  x1.x2 + y1.y2 + z1.z2
        \ z1   \ z2

Distance between M and P :

u = MP

Dist (M,P) = ||u|| = SquareRoot(u.u) = SquareRoot( (a2-a1)2 + (b2-b1)2 + (c2-c1)2 )

v = 0  <=>  ||v|| = 0  <=>  v.v = 0

   / a1     / a2               / a2-a1
 M | b1   P | b2      u = MP = | b2-b1
   \ c1     \ c2               \ c2-c1

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