**Review: **A
*central angle* of a circle is an angle with its vertex at
the center of that circle. In the diagram below, angle A is a
central angle. The measure of angle A equals the measure of arc
CD. (We say that angle A *intercepts* arc CD.)

**Question: **In
the diagram below, the measure of arc CD equals the measure of
arc EF. What is true about angles DAC and EAF? Why?

Construct a circle A, then construct four
points on the circle. Label these points C, D, E and F. Construct
chords CD and EF. Construct arcs EF and DC. (To construct arc
EF, click on point E, click on the circle, click on point F and
choose "Arc on Circle" from the Construct menu at the
top of the page. (By the way, we say that chord EF *subtends*
arc EF.) Measure lengths EF and DC, then measure arc angles EF
and DC. Drag various points until chords EF and DC are congruent.
What appears to be true? Proof? **Hint?**