We the diamond moderators have access to some aggregate voting statistics. I had a look. The total number of downvotes from any single user to you is so low that there is nothing actionable.
- Some users maintain a very strict standard about the material they want to see on the site, and downvote the posts that in their opinion fall short of that. This is very much their right as long as they don't systematically "hunt" your posts for the purpose of downvoting.
- Your most frequent downvoter is known for such standards. Others have complained about this, but no one has been able to refute them. In other words, this voter knows their math better than anyone who has complained.
- I checked the most recent downvotes. AFAICT your posts were not too good, and many of them had at least two downvotes. Because a single user can only cast a single vote on any post this is a sign that the problem is with your posts - they can be improved (or simply deleted, if you committed an error).
My advice would be that you may benefit from getting more familiar with what others expect from posts on the site. Most of the time we expect answers to also have pedagogical value, and posts consisting just of a calculation, even if correct, don't have much of that. I realize that English is not your first language, and it is not unnatural that you therefore seek to be as brief as possible. Unfortunately that also seems to make it very difficult to follow your line of thought. Brevity of expression actually makes the language difficulties worse. So:
- Don't be too afraid of committing language errors! I can easily pack several errors into a single post myself! This is an international site, and by and large we are very understanding of such difficulties.
- When answering a question think of yourself as a teacher rather than a student. You are not sitting in an exam here! You are trying to help the asker understand what's going on (rather than trying to convince your teacher that you have completed the necessary steps towards a solution). Not unlike when helping a fellow student. Not only how to do the necessary steps but also explaining why they want to take those steps.
- Study already answered questions within the tags that interest you. Try and figure out what makes a good answer. Warning: voting is not an entirely accurate guide here (I won't go into details here. Let me just state that there is too much variation in voting to completely trust it. The vote tallies of two answers given at nearly the same time do usually correlate with their quality, though.)