In the given diagram, O is the center of the circle, OBA = 20^, and OCA = 20^ extrm. Find the value of COB.

$O$ is the center of the circle.
The angle marked at $B$ is $\angle OBA = 20^\circ$.
The angle marked at $C$ is $\angle OCA = 20^\circ$.
We need to find the value of $\angle COB$.

In $\triangle OAB$, $OA = OB$ (both are radii of the circle).
Therefore, $\triangle OAB$ is an isosceles triangle.
The base angles of an isosceles triangle are equal, so $\angle OAB = \angle OBA$.
Given $\angle OBA = 20^\