2009*20012001-2001*20092009
=2009*(20012001-2001*10001)
=2009*2001*(10001-10001)
=2009*2001*0
=0
1/1*2+1/2*3+1/3*4+…+1/9*10
=1-1/2+1/2-1/3+1/3-1/4+…+1/9-1/10
=1-1/10
=9/10