78×79/78
=（79-1）×79/78
=79×79/78 -79/78
=78- 79/78
=77又79/1
2006×2005/2004
=（2005+1）×2005/2004
=2005×2005/2004 +2005/2004
=2004+2005/2004
=2004又2005/2004