# How can I use a large solidus in a display fraction in LyX?

I would like to use the equivalent of the latex \middle/ in a display equation in LyX. I have a fraction with a large left hand side, and I want the solidus (/) to be automatically sized to match the equation height.

What is the best way to accomplish this in LyX?

1. The simplest is to type \middle/. Corresponding empty left and right delimiters \left. \right. can be obtained from the Delimiters dialog.
2. A more advanced solution is to define a LyX macro with LaTeX output as above: \left.#1\middle/#2\right., and LyX display: \left.#1\big/#2\right. (which hides LaTeX command \middle which LyX does not know about). This will look a bit nicer inside LyX but still do what you asked for in LaTeX.
3. Assuming that you know which side is bigger (as you indicate in your question), you can use \left or \right instead of \middle. This allows you to see the variable height in LyX too. For this, select what corresponds to \left. \right/ (for the case where the left-hand side is bigger) from the Delimiters dialog. Correct horizontal spacing may require the use of enclosing braces.