设 1/2+1/3+1/4+1/5 = A
(1+1/3+1/4) = B
(1+1/2+1/3+1/4+1/5)*(1/3+1/4+1)-(1+1/3+1/4)*(1/2+1/3+1/4+1/5)
= 1 * B + A * B - B * A
= 1 * B
= (1+1/3+1/4)
= 19/12