20/11/15 21:47:54.81 WOfFn0Se.net
s(m,n; A,C,E) = ⊿FAB + ⊿BCD + ⊿DEF + ⊿BDF
= (1/2)mn{sin(A)+sin(C)+sin(E)} + ⊿BDF
第二余弦定理より
BD^2 = m^2 + n^2 - 2mn･cos(C),
DF^2 = m^2 + n^2 - 2mn･cos(E),
FB^2 = m^2 + n^2 - 2mn･cos(A),
⊿BDF = S(BD,DF,FB)
= (1/4)√{4(mm+nn