Tam giác EDC có góc E = 180 - D - C => KEB=(180 − D − C) / 2
góc ABE = 180 - B (kề bù)
EK giao AB = I
góc EIB = AIK (đđ) =180 - KEB - ABE = 180 − (180 − D − C) / 2 − 180 +B = B − (180 − D − C) / 2
Tam giác BMC: góc BMC = 180 - B - C => KMC = (180 − B − C) / 2
góc MDA = 180 - D
MK giao AD= J
góc AJK = MJD (đđ) = 180 - MDA - KMC
=180 − 180 + D− (180 − B − C) / 2=D− (180−B−C) / 2
Tứ giác AIKJ:
IKJ=360−A−AIK−AJK
=360−A−B + (180−D−C) / 2 − D + (180 − B − C) / 2
=720/2 − 2A/2 − 2B/2 + (180−D−C) / 2 − 2D/2 + (180 − B − C) / 2
= (1080 −3B−3D−2A−2C) / 2