Both are used. A single $ is used for inline math, the double $$ begins an equation on its own line. For example:
The solution to $ax^2 + bx + c = 0$ will be $$x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$$
The first block will be mid-sentence, while the second one will get its own line.
Both are used. A single $ is used for inline math, the double $$ begins an equation on its own line. For example:
The solution to $ax^2 + bx + c = 0$ will be $$x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$$
The first block will be mid-sentence, while the second one will get its own line.