20. 如圖,有兩個直角三角形 ABC、BDE,三內角分別為 30°-60°-90°、45°-45°-90°。已知
【用戶】Jiunndar Yeh
【年級】幼兒園下
【評論內容】在△BDE中,∠D = ∠BED = 45°,故BD = BE在△ABC中,∠EBC = 60°,又BC = BD = BC故在△BCE中,因BC = BE,得∠BCE = ∠BEC∠BCE + ∠BEC = 180° - ∠EBC = 180° - 60° = 120°⇒ ∠BCE = ∠BEC = 120°/2 = 60°故∠DEC = ∠DEB + ∠BEC = 45° + 60°= 105° ...選項(B)正確