Answer: A. 9
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Step-by-step explanation:
Draw in the segments AO and OC.
Triangle ABO is congruent to triangle CBO. We can prove this through the use of the HL theorem. HL stands for hypotenuse leg.
Since the triangles are congruent, this means the corresponding pieces AB and BC are the same length.
Then we can say:
AB+BC = AC .... segment addition postulate
AB+AB = AC .... plug in BC = AB
2*AB = AC
2*AB = 18
AB = 18/2 .... divide both sides by 2
AB = 9
In short, the chord AC is bisected by the perpendicular radius drawn in the diagram. So all we do is cut AC = 18 in half to get AB = 9.