Hello everybody,

I am stuck with a proof of which I think that I have an idea of how to solve it.

I have given two vectors with . I know that and .

Now I am supposed to show a couple of things and what I am left to do is to prove that there is only one s.t. and .

OK, now here is what I have done. I know from the Schwarz inequality that is possible in general if I chose the right z. I get this same equality if I set the given definition of d into my other constraint. Also should I be able to solve for x since I have one unknown z and three requirements for z. However, whenever I solve for z I get some nonsence. I am not exactly sure about what I am doing wrong or how else I can show that there is only one z for that all these requirements hold. Can one of you help me. Even a hint would be more than enough. Thanks a ton! Cheers.