(2012?黄冈) 如图,在△ABC中,AB=AC,∠A=36°,AB的垂直平分线交AC于点E,垂足为点D,连接BE,则∠EB

kuaidi.ping-jia.net  作者:佚名   更新日期:2024-07-05
如图,在△ABC中,AB=AC,∠A=36°,AB的垂直平分线交AC于点E,垂足为点D,连接BE,则∠EBC的度数

∵ AB = BC
∴∠BAC = ∠ ACB

在△ABC中,
∵∠A = 36°
∴根据三角形内角和定理,∠BAC = ∠ ACB = 72°
∵DE为AB的垂直平分线
∴AD = BD,∠ADE = ∠BDE = 90°

在△ADE与△BDE中,根据边边边定理
ED = ED
∠ADE = ∠BDE
AD = BD


∴△ADE ≌△BDE
∴∠A = ∠DBE
∴∠DBE = 36°
∵∠BAC = 72°
∴∠EBC = ∠BAC - ∠DBE = 36°

连接AM、AN、过A作AD⊥BC于D,∵在△ABC中,AB=AC,∠A=120°,BC=6cm,∴∠B=∠C=30°,BD=CD=3cm,∴AB=BDcos30°=23cm=AC,∵AB的垂直平分线EM,∴BE=12AB=3cm同理CF=3cm,∴BM=BEcos30°=2cm,同理CN=2cm,∴MN=BC-BM-CN=2cm,故选C.

∵DE是AB的垂直平分线,
∴AE=BE,
∴∠ABE=∠A=36°,
∵AB=AC,
∴∠ABC=∠C=
180°?∠A
2
=72°,
∴∠EBC=∠ABC-∠ABE=72°-36°=36°.
故答案为:36°.