题目内容
(本题满分12分)
如图,I是△ABC的内心,∠BAC的平分线与△ABC的外接圆相交于点D。BD与ID相等吗?为什么?(12)
如图,I是△ABC的内心,∠BAC的平分线与△ABC的外接圆相交于点D。BD与ID相等吗?为什么?(12)

解:BD=ID连接BI
∵AD平分∠BAC∴∠BAD=∠CAD
∵∠DBC=∠CAD
∴∠BAD=∠DBC
∵∠BID=∠BAD+∠ABI
∠DBI=∠DBC+∠CBI
∠ABI=∠CBI
∴∠BID=∠DBI
∴BD=ID

∵AD平分∠BAC∴∠BAD=∠CAD
∵∠DBC=∠CAD
∴∠BAD=∠DBC
∵∠BID=∠BAD+∠ABI
∠DBI=∠DBC+∠CBI
∠ABI=∠CBI
∴∠BID=∠DBI
∴BD=ID
试题分析:解:BD=ID连接BI

∵AD平分∠BAC∴∠BAD=∠CAD
∵∠DBC=∠CAD∴∠BAD=∠DBC
∵∠BID=∠BAD+∠ABI
∠DBI=∠DBC+∠CBI
∠ABI=∠CBI∴∠BID=∠DBI∴BD=ID
点评:本题难度中等。运用同弧的圆周角相等证明即可。
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