joplin/packages/app-cli/tests/html_to_md/mathjax_block.md

Block formulas are surrounded by double dollar signs. For example, $$x = \frac{-b \pm \sqrt{b^2 - 4ac} }{2a}$$ renders, on a separate line, as

x = \frac{-b \pm \sqrt{b^2 - 4ac} }{2a}.