有向數加減
Worksheets can be downloaded at
https://www.dropbox.com/s/flu1zuagin6k1xr/directed_num1_11-12.doc
http://www.geogebratube.org/material/show/id/13662 (Download GeoGebra file)
Worksheets can be downloaded at
https://www.dropbox.com/s/p4eva4bpjd1vjcs/directed_num2_11-12.doc