1/m(m+1)+1/(m+1)(m+2)+1/(m+2)(m+3) =[(m+2)(m+3)+m(m+3)+m(m+1)]/m(m+1)(m+2)(m+3) =(3m*m+9m+6)/m(m+1)(m+2)(m+3) =3(m+1)(m+2)/m(m+1)(m+2)(m+3) =3/m(m+3)
