To use inline math, put it between $...$. E.g.
Euler's formula is $e^{i\theta} = cos(\theta) + \iota sin(\theta)$.
is rendered as
Euler’s formula is e^{\iota\theta} = cos(\theta) + \iota sin(\theta).
To use display math, use two dollar signs like $$...$$. For example:
$$
\oint_C (L\, dx + M\, dy) = \iint_{D} \left(\frac{\partial M}{\partial x} - \frac{\partial L}{\partial y}\right) dA
$$
will render as
\oint_C (L\, dx + M\, dy) = \iint_{D} \left(\frac{\partial M}{\partial x} - \frac{\partial L}{\partial y}\right) dA
Also, as a tip, since Discourse supports HTML, you can use HTML entities for much of the math we need. Some useful entities:
| Entity | Result | Notes |
|---|---|---|
∃ |
∃ | |
∀ |
∀ | |
∈ |
∈ | |
¬ |
¬ | |
∧ |
∧ | |
∨ |
∨ | |
→ |
→ | There are other arrows, too, |
⇔ |
⇔ | Capitalizing the A doubles the arrow |
² |
² | Works for 1, 2, and 3 |
ℤ |
ℤ | -opf makes blackboard bold in general |
λ |
λ | All greek letters have an entity |
Γ |
Γ | Capitalizing the entity capitalizes the letter. |
There’s a full list here. You can also use the <sup /> and <sub /> html tags to superscript and subscript things respectively.