## Elementary Geometry for College Students (6th Edition)

30$^{\circ}$
If CD=OC and OC=OD, then CD=OD. Since all three sides are congruent, COD is an equilateral triangle. The altitude of an equilateral triangle splits it into two 30 60 90 triangles, therefore, m$\angle$COE is equal to 30$^{\circ}$. The measure of an arc is equal to the measure of the central angle that intercepts it, therefore arc CF=30$^{\circ}$.