I'm not sure if this the correct area but Ill go ahead anyways. I have three points (A, B, and C) in 3D space that form a triangle. All three points have the (x,y,z) coordinates defined. Using the dot product I have solved for the angle θ (or in other notation the angle ABC) of the triangle. I want to extend the magnitude of vector AC by a known amount and re-define the coordinate points of C. A picture is always worth a thousand words, so one is attached. I do know that you can define the magnitudes of AB and BC and then simply use the law of cosines to calculate the new angle but I am trying to determine this alternative method but maybe that is why it won't work. Thanks for any help. *edit* just slight clarification that I know how to calculate the magnitude of vector AC and since I know the magnitude of the extension CC' they can simply be added together. What I can't determine is the coordinate points of C' such that the magnitude of AC' equals AC + CC'.