2000*2000*（1/(2000*2001)+1/(2001*2002))
=2000*2000*(1/2000-1/2001+1/2001-1/2002)
=2000*2000*(1/2000-1/2002)
=2000*2000*(2/(2000*2002))
=2000*2000*2/2000/2002
=2000/1001