My question defines an area bounded by for sides, a quadrangle, I
suppose I should said "plane geometric shape" rather than "triangle",
sorry for the confusion.


Walt wrote:
OK. If you mean "polygon" please say "polygon".


Isn't a triangle a polygon?

Anyway, what is the point in using plane geometry to analyse
sails? They are not flat.

One way to maximize sail area would be to make the sails
more spherical, with boundaries defined by the limits of the
above-mentioned polygon.

DSK