Multi-line Latex formulas do not work

[Kelfvin]

I use a lot of Latex formulas in my blog. I found that Multi-line Latex formulas did not work.

I write a LaTeX code like this:

$$
\begin{align*}
Macro-F1 &= \frac{F1_{positive} + F1_{negative}}{2} \\
& = \frac{\frac{2TN}{2TN + FN + FP} + \frac{2TP}{2TP + FP + FN}}{2} \\
& = \frac{TP}{2TP + FP + FN} + \frac{TN}{2TN + FN + FP}
\end{align*}
$$

It should work like this✅:

But the actual display looks like this❌:

It renders the formula as one line instead of the multiple lines I wanted.

I have search on the Internet, somebody solve this problem by change \\ to \\\\ . I tried this approach and it worked, but I don't think it was handled elegantly enough.

Is there another way to handle this more elegantly? Can anyone help me with this? 🆘