Proving Vector Orthogonality of b onto a

[darthxepher]

Homework Statement

Show that vector orth of b onto a=b-proj of b onto a is orthogonal to a.


I totally don't know where to start :(

and I don't know hat orth of b onto a means...

 

[CompuChip]

and I don't know hat orth of b onto a means...

Well, that's kind of crucial for answering this question, isn't it. Do you have any lecture notes or textbook in which you can look this up?
Do you remember what the dot product ($\vec a \cdot \vec b$) meant geometrically?