While we try, we can not guarantee that all answers posted are correct.
There are several issues here:
Answers are not reviewed before they become visible to the outside world. Therefore, an invalid answer can easily show up.
Not all questions get reviewed. Some of us do check answers for validity but that does not mean all answers get checked. If you think an question might not be correct, you should flag it yourself so we can focus on those questions.
The objective of this site is more to demonstrate solution methods which would enable others to solve similar problems than to provide "the correct answer". If someone simply copies answers they find here (for example as homework answers) then they should ensure they understand the process and verify the results.
For some questions there is no single "correct" answer. Sometimes this is because of the way the question has been expressed and other times it is because there is no single answer (or way of obtaining an answer).