Is it possible that a high reputation (>50K) user post a completely wrong answer? or its answer contains some lines that shows the user is misunderstanding that topic in math?
If so, how we can trust that answer if nobody up-vote it?
Is it possible that a high reputation (>50K) user post a completely wrong answer? or its answer contains some lines that shows the user is misunderstanding that topic in math?
If so, how we can trust that answer if nobody up-vote it?
Yes. That is possible, and it certainly happens.
This is why you should usually be critical and vote after reading and understanding an answer.
And yes, sometimes people will vote a wrong answer because they trust the user. But hopefully someone will actually read it, and point out the mistakes. At least on my end as a high reputation user, that worked out pretty well every time I made a mistake (that I know of, anyway).
I'd like to tackle the second part of your Question:
If so, how we can trust that answer if nobody up-vote it?
The ideal for Math.SE is to have problems solved by reasoned mathematical argument. By this I mean the correctness of the solution should be evident by following the logic (reasoning) of the Answer. In particular we should avoid asking Readers to accept proof by authority.
Naturally a great variation occurs in the ability of Readers to follow arguments when the subject matter, and even the terminology, may be unfamiliar. This is one of the motivations for wanting context in the presentation of a Question, so that a willing Reader can try to respond in an accessible way to the OP. But in many cases a problem can be solved in various ways, and it is desirable to see that the lines of argument, done correctly, come in the end to the same conclusion.
My advice in posting Questions is therefore to ask about what you will be able to judge for yourself of its correctness. Pushing beyond this limitation may lead to a failure to learn the mathematics. So in such a case it is likely better to step back and ask a more fundamental "threshold" Question, to prepare for next steps in understanding.