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因為.CE,CF分別是三角形ABC的內角ACB的平分線和外角BCM的平分線 所以.角ECB=2分之1角ACB,角BCF=2分之1角BCM 因為.角ACB十角BCM=180度 所以.角ECB十角BCF=90度 即:角ECF=90度.
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