2*5/1+5*8/1+8*11/1+...+2006*2009/1+2009*2012/1 =3/1(2/1 -5/1 +5/1 -8/1+.+2009/1 -2012/1) =3/1(2/1 -2012/1) =3/1(2012/1005) =2012/335