Side note: PF is awesome! This is kind of like Tutor.com, only it's free and others can contribute their answers. 1. The problem statement, all variables and given/known data Explain why triangle ABD is similar to triangle ABC and then find the length of side AB. Angles B and C are congruent. m(ADB)+m(BDC) = 180 degrees. AD=4 and DC=12. 2. Relevant theorems, postulates, properties, etc. Corresponding parts of similar angles are similar. Symmetric property of similarity - if XYZ is similar to ABC, then XZY is similar to ACB, ZXY is similar to CAB, etc. 3. The attempt at a solution The similarity ratio is 4:16 (side length in ABD:side length in ABC).I know that. If I knew at least one side length I could easily figure out what AB is. The problem is I don't.