Statement 1: AM is the midpoint of AB
Reason 1: Given
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Statement 2: AM+MB = AB
Reason 2: Segment Addition Postulate
Further explanation: Basically this idea is where you can break up the line segment into smaller pieces, or glue smaller segments to form a larger segment.
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Statement 3: AM = MB
Reason 3: Definition of midpoint
Further explanation: The midpoint cuts a line segment in half. The two smaller pieces AM and MB are congruent (ie equal in length)
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Statement 4: AM + AM = AB
Reason 4: Substitution
Further explanation: We replace the MB in statement 2 with AM. This replacement is possible because of the equation AM = MB in statement 3.
This concludes the proof.