答:1003/2005
(1-1/2*1/2)*(1-1/3*1/3)*.*(1-1/2005*1/2005)
=(1-1/2)*(1+1/2)*(1-1/3)*(1+1/3)*(1-1/4)*(1+1/4)2003/2004*2005/2004*2004/2005*2006/2005=1/2*2006/2005=1003/2005