Zettlr supports writing math and formulae using the KaTeX library. In order to use math, enclose your formulae with dollar signs (
$). You can also use so-called "display math" by using two dollar signs instead of single dollar signs to enclose your formulae.
You can also define math blocks by putting two dollar signs on a single line before and after the math block.
While the syntax highlighting of math is monospaced font, you can also tell Zettlr to render the math blocks in place. For this to work, make sure you have activated the corresponding option in the display settings.