Finding a side length in similar triangles?

[moonman239]

Side note: PF is awesome! This is kind of like Tutor.com, only it's free and others can contribute their answers.

Homework Statement

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.

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.

 

[Mark44]

Side note: PF is awesome! This is kind of like Tutor.com, only it's free and others can contribute their answers.

Homework Statement

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.

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.

It would be helpful if you could post a screen shot or sketch of the two triangles.