3/1*5 +3/5*9+ 3/9*13+ … +3/2005*2009
=3/4*(1-1/5+1/5-1/9+1/9-1/13+……+ 1/2005-1/2009)
=3/4*(1-1/2009)
=3/4*2008/2009
=1506/2009/
3/1*5 +3/5*9+ 3/9*13+ … +3/2005*2009
=3/4*(1-1/5+1/5-1/9+1/9-1/13+……+ 1/2005-1/2009)
=3/4*(1-1/2009)
=3/4*2008/2009
=1506/2009/