① tan α = tan (∠ABC)/2 = b/(a+c)
② r = IE = BE tan α
③ AG = AB - BG = AB - BE = c - r/tan α
④ tan β = IG/AG = r/AG
⑤ tan (α + β) = (tan α + tan β)/(1 - tan α tan β)
⑥ α + β + γ = 90°なので tan γ = 1/tan (α + β)
⑦ r = ED tan γ
⑧ BD = BE + ED
  AD = AF+FD = AG + ED
  CD = BC - BD
⑨ 2R = (AB+BD+DA)r, 2S = (AD+DC+CA)s
①、②より BE = r(a+c)/b
①、③より AG = (bc-r(a+c))/b
④より tan β = br/(bc-r(a+c))
⑤より tan (α + β) = b2c/((a+c)bc-r((a+c)2+b2))
(a+c)2+b2 = (a+c)2+(a2-c2) = 2a(a+c) なので
tan (α + β) = b2c/((a+c)(bc-2ar)) = (a-c)c/(bc-2ar)
⑥、⑦より ED = (a-c)cr/(bc-2ar)
⑨より
2R = (AB+BD+DA)r = (2AB+2ED)r
 = 2( c + (a-c)cr/(bc-2ar))r
 2cr(bc - (a+c)r)/(bc - 2ar)
2S = 2(R+S) - 2R
 = bc - 2cr(bc - (a+c)r)/(bc - 2ar)
 = (b2c2 -2(a+b)bcr + 2c(a+c)r2)/(bc - 2ar)
 = (a+c)c(c(a-c)-2br+2r2)/(bc - 2ar)
また
AD + DC + CA = AF + FD + DC + CA
  = AG + ED + DC + CA
  = AB - BD + BC - BE + CA
  = (a+b+c) - 2BE
  = (a+b+c) - 2r(a+c)/b
  = (b(a+c)+b2-2r(a+c))/b
  = (a+c)((a+b-c) - 2r)/b
なので⑨より
s = bc(c(a-c)-2br+2r2)/((bc - 2ar)((a+b-c) - 2r))
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