原式=1/2×(1/2-1/4+1/4-1/6+1/6-1/8+……+1/2006-1/2008)
=1/2×(1/2-1/2008)
=1/2×1003/2008
=1003/4016
1/(2×4)=1/2×(1/2-1/4)