= \(2005\cdot2005-\left(2005-1\right)\left(2005+1\right)\)
= \(2005\cdot2005-\left(2005\cdot2005+2005-2005-1\right)\)
= \(2005\cdot2005-2005\cdot2005+1\)
= 1
2005*2005-2004*2006
=2005*2005-2004*(2005+1)
=2005*2005-2004*2005+1
=2005*(2005-2004)+1
=2005*1+1=2006