Question

The origin of two vectors is at the point (0,0,0) of a system of rectangular coordinates. If their terminal points are at (1, 2, 3) and (3, 2, 1) respectively. What is the angle between the two vectors?

Answer 1

In geometry, Keller's conjecture is the conjecture that in any tiling of Euclidean space by identical hypercubes there are two cubes that meet face to face. For instance, as shown in the illustration, in any tiling of the plane by identical squares, some two squares must meet edge to ed.

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