а) 7/12+5/18=б)15/16-3/4=в)-3/14+2/21=​

а) 7/12 + 5/18 = (73)/(123) + (52)/(182) = 21/36 + 10/36 = 31/36

б) 15/16 - 3/4 = (151)/(161) - (34)/(44) = 15/16 - 12/16 = 3/16

в) -3/14 + 2/21 = (33)/(143) + (22)/(212) = -9/42 + 4/42 = -5/42