Issue After posting my first article, I found that seems MathJax “cannot” parse the newline symbol (\\) properly.
Usually, we do latex like
1 2 f(x)=ax+b\\ g(x)=kx+d However, it does not work.
Solution After checking the source code generated by Hugo, I found that seems all \\ would be replaced by \. I believe it was done by a kind of converting process.
So the solution is quite easy: replace all \\ to \\\\.