For some reason the mathjax blocks aren't working in preview on the main site.
$$
5=\sqrt{25}
$$
Inline math is ok, though: $5=\sqrt{25}$
Here's an example directly from the math.stackexchange.com markdown help:
The Gamma function satisfying $\Gamma(n) = (n-1)!\quad\forall
n\in\mathbb N$ is via through the Euler integral
$$
\Gamma(z) = \int_0^\infty t^{z-1}e^{-t}dt\,.
$$
This works, though. Guess I need to understand what's wrong with my latex math in the other answer, then... Looks like blank lines are a problem. But then if you have a series of equations, they all appear on one line:
$$
V = IR
I = \frac{V}{R}
P = VI
P = I^2R
I = \sqrt{\frac{P}{R}}
$$
So the answer is that each individual equation has to be delimited?
$$
V = IR
$$
$$
I = \frac{V}{R}
$$
$$
P = VI
$$
$$
P = I^2R
$$
$$
I = \sqrt{\frac{P}{R}}
$$
Somewhat defeats the purpose of the block, it seems. Guess I need to find out what latex requires for a new line.
$$
V = IR\\
I = \frac{V}{R}\\
P = VI\\
P = I^2R\\
I = \sqrt{\frac{P}{R}}
$$
And the answer is that each line may be split with two backslashes: \\