1*5分之3+5*9分之3+9*13分之3+...+1997*2001分之3
=3/4*(1-1/5+1/5-1/9+1/9-1/13+……+1/1997-1/2001)
=3/4*(1-1/2001)
=3/4*2000/2001
=500/667
1*5分之3+5*9分之3+9*13分之3+...+1997*2001分之3
=3/4*(1-1/5+1/5-1/9+1/9-1/13+……+1/1997-1/2001)
=3/4*(1-1/2001)
=3/4*2000/2001
=500/667