Ubuntu: How to enable spellchecker in LyX

1. Open the Terminal and install Enchant.
   
   sudo apt-get install enchant
2. Open LyX and select Tools > Preferences > Language Settings > Spellchecker
3. Select Enchant for Spellchecker Engine.
4. Click on Apply, Save and finally Close.

LaTeX: How to add vertical bar with evaluation or limit

$$\frac{\partial f}{\partial x\partial y}\bigg\vert_{x=y=0}$$