#### Answer

The proof is in the following form:
Column 1 of proof; Column 2 of proof.
Thus, we have:
1. ADC is a right angle, and CDB is a right angle; Given
2. The altitude of triangle ABC is CD; definition of an altitude
3. CDA is similar to triangle ACB; the altitude of the hypotenuse of a right triangle forms a right triangle similar to the original right triangle.
4. $\frac{AB}{AC} = \frac{AC}{AD}$; CSSTP