證明：?$[$?模塊探究?$]$?延長(zhǎng)?$BO$?交?$AC$?于?$D$?
∵?$∠BOC=∠C +∠CDO，$??$∠CDO=∠A+∠B$?
∴?$∠BOC=∠A+∠B+∠C$?
?$(2)∠A，$??$∠B，$??$∠C，$??$∠D，$??$∠E$?的度數(shù)之和是?$180°$?
證明：∵?$∠BOC=∠A+∠B+∠C，$??$∠COD=∠E+∠D$?
∴?$∠A+∠B+∠C+∠E+∠D=∠BOC+∠COD=180°$?
?$(3)$?∵?$∠DMN=∠G+∠GNM，$??$∠GNM =∠BNC=∠F+∠B+∠C$?
∴?$∠DMN=∠G+∠F+∠B+∠C$?
∵?$∠EMD=∠A+∠E+∠D$?
∴?$∠A+∠E+∠D+∠G+∠F+∠B+∠C = ∠EMD +∠DMN=180°$?
