1/(a+1)(a+2)+1/(a+2)(a+3)+...+1/(a+2010)(a+2011)
= 1/(a+1)-1/(a+2) + 1/(a+2)-1/(a+3) + .+1/(a+2010)-1/(a+2011)
= 1/(a+1)-1/(a+2011)
= 2010/[(a+1)(a+2011)]