求角bdc=90度-1/2角a,

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求角bdc=90度-1/2角a,

求角bdc=90度-1/2角a,
求角bdc=90度-1/2角a,
 

求角bdc=90度-1/2角a,
∠EBC=∠A+∠ACB
∠FCB=∠A+∠ABC
∠DBC=1/2∠EBC=1/2∠A+1/2∠ACB
∠DCB=1/2∠ECB=1/2∠A+1/2∠ABC
∠DBC+∠DCB+∠D=180°
1/2∠A+1/2∠ACB+1/2∠A+1/2∠ABC+∠D=180°
1/2∠A+1/2(∠ACB+∠A+∠ABC)+∠D=180°
∠ACB+∠A+∠ABC=180°
1/2∠A+1/2*180°+∠D=180°
∠D=90°-1/2∠A