{\displaystyle S=\iint \limits _{A}{\sqrt {\left({\frac {D(x,y)}{D(u,v)}}\right)^{2}+\left({\frac {D(y,z)}{D(u,v)}}\right)^{2}+\left({\frac {D(z,x)}{D(u,v)}}\right)^{2}}}\;\mathrm {d} \,u\,\mathrm {d} \,v}