15.7 - ( 5.7 - 2.8 ) = 15.7 - 5.7 + 2.8 = 12.84/9 - 7/16 * 4/9 = 4/9 * 1 - 7/16 * 4/9 = 4/9 * (1 - 7/16) = 4/9 * 9/16 = 1/4（3/4 - 5/7 ）/ (1/56) =( 3/4 / 1/56 ) - ( 5/7 /1/56 ) = 42 - 40 = 2望采纳
