This was asked at math.stackexchange.com on Nov 7 2014.<br> This was migrated to mathoverflow.net (here) on Nov 20 2014.<br> A similar question was asked at math.stackexchange.com on Nov 30 2015.<br> That one generates the curve by simple linear interpolation between the 4 points, and doesn't say whether the result is "minimal".<br> Something like an answer was added on Nov 30 2015. (It is not an expression in terms of a, b, c, d.)<br> **Compute the area defined by four non-planar points**<br> math.stackexchange.com/questions/1552551/…