how to find the length of a line segment given a congruent line segments length and the sum of both.

here is line ac. b is its midpoint.
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a b c
ab = 12 - (4/2) we know that ab + bc = ac (segment addition postulate).
ac = 20 so 12 - (4/2) + x = 20.
find bc. solve the algebraic equation. side note: we also know
that ab = bc because they
are congruent sides of a
midpoint.