因∠ABC=∠C
∠A=180-∠ABC-∠C=180-2∠C
∠ABD=180-∠A-∠ADB=180-2∠A
=180-2(180-2∠C)
=4∠C-180
∠ABC=∠ABD+30=4∠C-180+30=∠C
5∠C=150
∠C=50
∠BDC=180-30-50=100度
因∠ABC=∠C
∠A=180-∠ABC-∠C=180-2∠C
∠ABD=180-∠A-∠ADB=180-2∠A
=180-2(180-2∠C)
=4∠C-180
∠ABC=∠ABD+30=4∠C-180+30=∠C
5∠C=150
∠C=50
∠BDC=180-30-50=100度