1. ΔACD∽ΔCBD∽ΔABC 1.If an altitude is drawn to the hypotenuse of a given right triangle, the two right triangles formed are similar to each other and to the given triangle. 2. 2. Corresponding parts of similar triangles 3. AC² = AD×AB 3. Product of means equal product of extremes (in 2) 4. 4. Corresponding parts of similar triangles 5. BC² = BD×AB 5. Product of means equal product of extremes (in 4) 6. AC²+BC² = AD×AB+BD×AB 6. Equals added to equals gives equals 7. AC²+BC² = AB×(AD+BD) 7. Distributive property 8. AD+BD = AB 8. A whole equals the sum of its parts. 9. AC²+BC² = AB×AB 9. Substitution 10. AC²+BC² = AB² 10. AB×AB = AB² Edwin