Question 123804
I usually hate Geometry proofs, but this one is fun!  Since CD is an altitude, it meets AB perpendicularly, forming 2 right angles.  So it splits triangle ABC into two right triangles.  Since triangle ABC is isosceles, angle A is congruent to angle B (call that measure x.  So both of these triangles have 1 angle measuring 90 degrees and 1 measuring x degrees.  So the remaining angle must be 180-(90+x), or (90-x)degrees.  So angle DCB is congruent to angle DCA, so CD bisects angle ACB.  If you need a formal proof, you need to look in your book and find those theorems and definitions for wording and details.  (Now I remember... that is the part I don't like!)  Good luck!