自主学习
【例1】
∵ AD、BE是∠CAB与∠CBA的平分线
∴ ∠1 = 1/2∠BAC,∠2 = 1/2∠ABC
∴ ∠1 + ∠2 = 1/2(∠BAC + ∠ABC) = 45°
而∠ADE = ∠1 + ∠2 故,∠ADE = 45°
【例2】∵ ∠CEF = ∠AED = 48°,∠BCA = ∠CEF + ∠F,
∴ ∠F = ∠BCA - ∠CEF = 74° - 48° = 26°,
∴ ∠BDF = 180° - ∠B - ∠F = 180° - 67° - 26° = 87°.
互动探究
1、D
2、∠HAG = 1/2∠ACB,
理由如下:如图11 - 5
∵ AD、BE、CF是△ABC的角平分线,
∴ ∠2 = 1/2∠ABC,∠1 = 1/2∠BAC,∠3 = 1/2∠ACB,
∵ ∠ABC + ∠BAC + ∠ACB = 180°,
∴ ∠1 + ∠2 + ∠3 = 90°,
∵ ∠AHG = ∠1 + ∠2,
∴ ∠AHG = 90° - ∠3,
∵ AG ⊥ BE,∴ ∠HAG = 90° - ∠AHG,
∴ ∠HAG = ∠3,
∴ ∠HAG = 1/2∠ACB.