5x/1*2+5x/2*3+……5x/2004*2005
=5x(1/1*2+1/2*3+……1/2004*2005)
=5x[(1/1-1/2)+(1/2-1/3)+(1/3-1/4)+……+(1/2003-1/2004)+(1/2004-1/2005)]
=5x(1-1/2005)
=2004x/401