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I want a triangle composed of points A, B and C in Cartesian 3D space.

I currently know the positions of points A and B, but I need point C. I have the line segment AB, and thus its magnitude. I have only the magnitudes of line segments AC and BC.

From this data, how do I derive point C? Please explain your logic. Thank you for your help.

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    its only defined up to a rotation...you can use the law of cosines to get the angles2011-03-25

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The lengths of segments AB, AC and BC are not enough to uniquely identify a triangle in 3-d space.

You can just pick a random plane that contains AB and make your triangle there.

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    @Henry: yes, but then it's just a matter of sign-choosing.2011-03-25
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The locus of point C is a circle (with a center inside AB) or nothing (depends on magnitudes of segments AC and BC).