Ming Jiang schrieb:
I want to insert text in a multiline equation, for example,a = 1, if some condition is met 2, if some condition is met like \text{if some condition is met} in TeX. How can I do this in LyX?
Use the shortcut "Alt+M M" which inserts math text. For more info about LyX and formula typesetting also have a look at LyX's math manual that you find in the Help menu.
regards Uwe
