1/1×3+1/3×5+1/5×7+……+1/17×19
=1/2(1/1-1/3+1/3-1/5+1/5-1/7+……+1/17-1/19)
=1/2(1-1/19)
=1/2×18/19
=9/19
1/1×11+1/11×21+1/21×31+……+1/2001×2011
=1/10(1/1-1/11+1/11-1/21+1/21-1/31+……+1/2001-1/2011)
=1/10(1-1/2011)
=1/10×2010/2011
=201/2011