$\displaystyle f(x) = a_3 x^3$
results in an invalid equation error.
$\displaystyle f(x) = a_3 x^3$
I've had some problems with the TEXtags. I think on my computer the best way is to write the equation etc. and then mark the equation and press on the sigmabutton in advanced mode to wrap the equation with the TEXtags.