maths-circle
2011-02-02 2:40 am
圖片參考：http://img138.imageshack.us/img138/4350/83432777.jpg
In the figure, chords CD and BF are produced to meet at E, where∠BEC=30°. Chords CB and DF are produced to meet at A, where∠CAD=40°. Find ∠DFB.
回答 (1)
2011-02-02 2:54 am
✔ 最佳答案
ㄥECA + ㄥEBC + 30° = 180° .....(1)ㄥECA + ㄥCDA + 40° = 180° .....(2)(1)+(2) :2ㄥECA + (ㄥEBC + ㄥCDA) + 70 = 3602ㄥECA + 180 + 70 = 360ㄥECA = 55°∠DFB = 180 - 55 = 125°
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