Don't specify HTTPS mathjax CDN address because it might change soon
https://groups.google.com/d/msg/mathjax-users/cPD_EI_lFAw/ovo4Bv5pidoJ

Pandoc's HTTP default, while less secure, is unlikely to break and will hopefully change to new https address if it emerges as a future-proof option.
Use \lim in example display formula to demonstrate display-style limits.
(Also fix inconsistent differentiation variable)