Write your equation using LaTeX and an Online Equation Editor
f(x)=int_{-infty}^x e^{-t^2}dt
Right click the image and copy the URL
http://www.codecogs.com/eq.latex?f(x)=int_{-infty}^x%20e^{-t^2}dt
Insert a picture using the image URL
To preserve the equation in a reusable format, it is recommended to include a copy of the source LaTeX.
The same method can be used for any medium that supports embedded images (blog, forum, email, etc.)
Google Docs will mirror the graphic in the document and will be viewable even if the online equation editor is inaccessible.
