I recently answered this question copying a solution directly from WolframAlpha. I want to know if this method is considered valid here on MSE because there're similar problems that can be solved directly using WolframAlpha. Did I do something wrong? Should I have solved the problem myself?
There is nothing wrong with using W|A, but you should definitely attribute the proof properly, and perhaps make it Community Wiki.
Perhaps not always a good idea. In principle there is nothing to say against it, and actually I used to post WA-links myself here. But WA may change its policies, so the behave of the freely available computing power. It appeared recently, that the same line of code, which gave a closed form solution a year ago doesn't behave the same way now in the openly available mode. (It was an analytical result for an infinite sum being equal zero then and now doesn't fully evaluate symbolically, requests for more computing time, but that's then a question of "professional mode" = money - here is my question where I used that WA-feature).
So one should consider such different behave in any question as well over the time (for some commercial and not mathematical reason) and I'm not going to use it for any essential result/proof/example anymore.