2×4分之1＋4×6分之1＋6×8分之1＋...＋2011×2012分之1
=1/2*(1/2-1/4+1/4-1/6+.+1/2010-1/2012)
=1/2*(1/2-1/2012)
=1/2*1005/2012
=1005/4024