78*(1/24-1/54)+24 *（1/54+1/78）-54*（1/24-1/78）
=78*1/24-78*1/54+24 *1/54+24*1/78-54*1/24+54*1/78
=（78-54）*1/24-（78-24）*1/54+（24+54）*78
=24*1/24-54*1/54+78*1/78
=1-1+1
=1