2005*2006-1 (2004+1)*2006-1 2004*2006+2006-1 2004*2006+ ( 2006-1 ) 2004*2006+2005
--------------------------- = -------------------------------- = ---------------------------------------- = ----------------------------------= -----------------------------
2006*2004+2005 2006*2004+2005 2006*2004+2005 2006*2004+2005 2006*2004+2005
=1 ( tử số = mẫu số )