a=1 ,b=2你已算出了
2)
=1/1×2+1/2×3+1/3×4+1/4×5+……+1/2011×2012
=1-1/2+1/2-1/3+1/3-1/4+……+1/2011-1/2012
=1-1/2012
=2011/2012