Are you assuming the four corner points are on the surface of the cone? If yes, then to determine the surface of the cone, you need four parameters: (x,y,z) of the tip, and the slop k. You are given four points, which would count as four conditions in generic position. But since you don't know the relative units of the xy and z, you have more variables, can you assume the xy are in the same unit and are proportional to the z? then you have only one additional variable, so another point should be enough.

In reply to Re: OT:Math problem: Grids and conical sections. by johnnywang
in thread OT:Math problem: Grids and conical sections. by BrowserUk

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