The radius of circle O is 18, and OC = 13. Find AB. Round to the nearest tenth, if necessary. (The figure is not drawn to scale.)
A. 12.4
B. 3.8
C. 24.9
D. 44.4

Question

The radius of circle O is 18, and OC = 13. Find AB. Round to the nearest tenth, if necessary. (The figure is not drawn to scale.) 
A. 12.4 	
  B. 3.8 	
  C. 24.9 	
  D. 44.4

anonymous · Answer

https://study.ashworthcollege.edu/access/content/group/45b8c516-1008-46d7-aa1d-bb9b62c786ff/geometry_exam_12_files/mc005-1.jpg

faiqraees · Answer

$$OB^2=BC^2+OC^2$$
$$BC^2=OB^2- OC^2$$
$$AB = 2BC$$

kamibug · Answer

Since the radius is the same all around a circle, that means that the length of line CB is going to be the same as AC. So all you have to do is find the length of CB and multiply it by 2 to get AB. :)
You can use the Pythagorean Theorem as @FaiqRaees suggests. 
We are given that OC is 13 and OB (the hypotenuse) is 18, so plug that in...
$$13^2 + CB^2 = 18^2$$
What do you get for CB? :)