1. (a-2)(a-3)+5a-42
=a²-5a+6+5a-42
=a²-36
=(a-6)(a+6)
2. x^2-(y^2-2y+1)
=x^2-(y-1)^2
=(x-y+1)(x+y-1)
1. 2009^2-2008*2010
=2009^2-(2009-1)(2009+1)
=2009^2-(2009^2-1)
=2009^2-2009^2+1
=1
2. 999^2+999*2+1
=(999+1)^2
=1000^2
=1000000