Merge pull request #1 from okapies/feature/new-mathjax-cdn

change to use alternative mathjax hosting

Fixed a typo

Update 23.html

Update 23.html

Add 23.html

Update 22.html

Add 22.html

Update 21.html

Add 21.html

Add 21.html

Update prog20-3.py

Fix example

Change font-size

Fix typo