test math

Maths between dollars is inline: $\sum_{k=1}^n k = \frac{n(n+1)}{2}$.

Maths between slash-square-brackets is display:
\[\sum_{k=1}^n k = \frac{n(n+1)}{2}\]

$$\sum_{k=1}^n k = \frac{n(n+1)}{2}$$