Based on the definition of an angle bisector, there is an erro in line 1, because, BC is an angle bisector.
<h3>What is an Angle Bisector?</h3>
An angle bisector is a line segment that bisects an angle into congruent parts.
In the proof given, since C is the incenter of the isoscles triangle ABD, therefore, line segment BC bisects ∠ABD.
This implies that BC is an angle bisector of ∠ABD not altitude.
Therefore, based on the definition of an angle bisector, there is an erro in line 1, because, BC is an angle bisector.
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