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