I am troubled by this. The authors of the book explicitly ask others not to post solutions to the exercises in their book on the web (Arora and Barak, "Computational Complexity", page vi):
We ask that you do not publish solutions for the book’s exercises on the web though, so other people can use them as homework and exam questions as well.
Should we have a different policy regarding answering this kind of situations (where the authors of the questions explicitly ask others not to post answers to their questions)?