1/x(x+3)+1/（x+3)(x+6)+...+1/(x+27)(x+30)
=1/3[(1/x)-1/(x+1)+1/(x+1)-1/(x+6)+...+
1/(x+27)-1/(x+30)]
=1/3[(1/x)-1/(x+30)]
=10/[x(x+30)]