Reflexive Property of Congruence
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The midpoint divides a line segment into congruent parts.
The mid-point
It is its midpoint
Basically the definition of bisect is to separate two parts of a line segment to create two congruent line segments, which leads to them being equal.
Let D represent the point on BC where the bisector of A intersects BC. Because AD bisects angle A, angle BAD is congruent to CAD. Because AD is perpendicular to BC, angle ADB is congruent to ADC (both are right angles). The line segment is congruent to itself. By angle-side-angle (ASA), we know that triangle ADB is congruent to triangle ADC. Therefore line segment AB is congruent to AC, so triangle ABC is isosceles.