2011×(1- 1/2)×(1- 1/3)×······×(1- 1/2010)×(1-1/2011)
2011×(1- 1/2)×(1- 1/3)×······×(1- 1/2010)×(1-1/2011)
=2011× 1/2 × 2/3 ······×2009/2010 ×2010/2011
=1
2011×(1- 1/2)×(1- 1/3)×······×(1- 1/2010)×(1-1/2011)
2011×(1- 1/2)×(1- 1/3)×······×(1- 1/2010)×(1-1/2011)
=2011× 1/2 × 2/3 ······×2009/2010 ×2010/2011
=1