唔識得計 圖形數學題

第一張
∠ABC = 180-∠BAD (interior angle)
∠BAD = 180-∠ADC (interior angle)
Therefore, ∠ABC = 180 - (180-∠ADC) = ∠ADC

第二張
extend the line CB to a new point G on the line DE so that GBC forms a straight line. Then
∠DEF = ∠ABC = ∠DGC (corr. angle)
Since ∠DEF =∠DGC, BC//EF (corr. angle)

第三張
∠BAC = ∠DCA (alt. angle, AB//DC)
∠EBA = 180 - ∠BAC = 180-∠DCA = ∠DCF (EACF是一條直線)

2007-04-12 12:11:16 補充：
corr. angle = corresponding anglealt. angle = alternate angle