Ahh this is a good one. Here we can use the concept of cyclic quadrilateral that we just learned yesterday.
A cyclic quadrilateral is a quadrilateral for which a circle can be circumscribed so that it touches each polygon vertex.
The opposite angles of a cyclic quadrilateral sum to 180 degrees.
In other words, angel ABD + angel ACD=180
Also since angel ABD+angel ABO=180
Therefore angel ABO=angel ACD
Now we have triangle AOC is similar to triangle DOB
From here you'll get the OA explanation that says AO * BO = OC * OD and thus solve for OC.
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