Originally Posted by

**IMK** Hello, was wondering if someone could help me with a little vector maths problem please.

What I need to find is the angle between two vectors that are derived from/relative to a third vector.

So what I having working/understood is that theta = cos-1(“vector a” * “vector b”) where “vector a” and “vector b” are 3d vectors at 90 deg to each other, x,y,z.

Basically the cos of the dot product of the two vectors…

What I now wish to do is to offset “vector a” and “vector b” by another vector, “vector c” and I have tried the following with mixed results.

(1) “vector a” = “vector a” - “vector c”, “vector b” = “vector b” - “vector c”

this returns an answer that is obviously incorrect.

(2) “vector a”= sqrt( “vector a” * “vector c”), “vector b” = sqrt(“vector b” *“vector c”), this returns an answer that seems to be correct as I can swap around the x,y,z values any of the vectors and the result seem to be consistent.

However I have no way of really proving the result, so can you help please?

Many thanks in advance IMK