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設A落在內部的點為G 在△ABC中,∠B+∠C=180-∠A 在△GDE中,∠GDE+∠GED=180-∠G=180-∠A 在四邊形BCED中,∠1+∠2=360-∠B-∠C-∠GDE-∠GED=2∠A
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