Try Html.fromHtml
instead. Simple and precise:
outputText.setText(Html.fromHtml("<sup>" + num + "</sup>/<sub>" + den + "</sub>"));
Note: You may need to give some height to the textview else the output will be clipped.
Alternatively, you could try this, where the number size is smaller:
outputText.setText(Html.fromHtml("<sup><small>" + num + "</small></sup>/<sub><small>" + den + "</small></sub>"));