原式=(1-1/2)×(1+1/2)×(1-1/3)×(1+1/3)×……×(1-1/2005)×(1+1/2005)
=1/2×3/2×2/3×4/3×……×2004/2005×2006/2005
=1/2×2006/2005
=1003/2005