



 
This seems to be a difficulty. The sixth edge is independent of the other five, except in the extreme case(s) where the four points are coplanar. Think of the five sides forming two triangles, hinged along the 'side' opposite the missing side. This hinge is flexible and and rotation the triangles around this hinge changes the missing side length. There can be no formula  regardless of the relationships among the other five. Walter Whiteley  


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