Why doesn't [tex] show formulas in preview?

[Ragualramen]

I'd like to write a questions but I don't understand how to write formulas, why if I write [ tex ] \phi[ / tex ] the preview doesn't show me any formula? thank you
Is it ok if I attach a photo of the formulas instead of writing them?

 

[PeroK]

Try this:

https://www.physicsforums.com/help/latexhelp/

 

[Ragualramen]

Ok i have to write $$ before and after

 

[DrGreg]

if I write [ tex ] \phi[ / tex ] the preview doesn't show me any formula

It does work if you omit all five of the spaces within the brackets.

 

[PeroK]

Ok i have to write $$ before and after

Or double hashes for inline formulas. E.g.

With $a = 1$ we have:
$$x = \frac{-b \pm \sqrt{b^2 -4c}} {2}$$