Why doesn frac work in equations?
It does, but only in LATEX documents because \frac is not a plain TEX command. The document you are presenting to TTH doubtless has no \documentclass command and other LATEX blurb at the top. If you insist on having LATEX commands available in such a document, you can use the -L switch. But note that other changes in interpretation (e.g. in footnotes) are implied by using this switch to tell TTH that this is a LATEX file.
