实践与探索!如图,△ABC中,∠ABC与∠ACB的平分线交于点I,根据下列条件,求∠BIC的度数, ①若∠ABC=40°,∠ACB=60°,则∠BIC=_; ②若∠ABC+∠ACB=80°,则∠BIC=_; ③若∠A=120°,则∠BIC=

问题描述:

实践与探索!如图,△ABC中,∠ABC与∠ACB的平分线交于点I,根据下列条件,求∠BIC的度数,

①若∠ABC=40°,∠ACB=60°,则∠BIC=______;
②若∠ABC+∠ACB=80°,则∠BIC=______;
③若∠A=120°,则∠BIC=______;
④从上述计算中,我们能发现∠BIC与∠A的关系式,并加以证明.

①∵∠ABC=40°,∠ACB=60°,∠ABC与∠ACB的平分线交于点I,∴∠IBC=20°∠ICB=30°,∴∠BIC=180°-∠IBC-∠ICB=130°;②∵∠ABC+∠ACB=80°,∠ABC与∠ACB的平分线交于点I,∴∠IBC+∠ICB=12(∠ABC+∠ACB)=40...