原式=1/3*（1-1/4)+1/3*(1/4-1/7)+1/3*(1/7-1/10）+……+1/3*(1/2008-1/2011）
=1/3*(1-1/4+1/4-1/7+1/7-1/10+……+1/2008-1/2011）
=1/3*(1-1/2011)
=670/2011