=1/2 * ( 1/5*6-1/6*7 + 1/6*7-1/7*8 + 1/7*8-1/8*9 ) =1/2 * ( 1/5*6-1/8*9) =1/2 * 7/360 =7/7201/a-1/b=(b-a)/ab,反之,(b-a)/ab=1/a-1/b 1/ac-1/bc=(b-a)/abc
