=(1+1/2)(1-1/2)(1-1/3)(1+1/3)(1+1/4)(1-1/4)……(1+1/2011)(1-1/2011) =1/2*3/2*2/3*4/3*3/4*5/4……*2011/2010*2010/2011*2012/2011 =1/2*2012/2011 =1006/2011