A Similarity problem

[Misr]

Hello there ,
http://img8.imageshack.us/img8/3118/circlem.jpg [Broken]

As you see the diameter = 5cm and AD = 4cm
the required is to find AB
it seems quite easy ,its just 6cm. I can solve it but when supposing that AB is a tangent (and this is not given in the problem) also i can't make use of the right angle opposite to the radius of this circle , so there must be something wrong with my answer.

so ,can u help?
Thanks in advance

 

[tiny-tim]

Hello Misr!

… I can solve it but when supposing that AB is a tangent (and this is not given in the problem) …

(hmm … not exactly to scale, is it? )

I'm sure AB is intended to be a tangent …

otherwise, B could be anywhere! ​

 

[Misr]

Yes I think so too
its just tangent

Thanks for help .