а) 5/8+1/4+7/12 = 15/24+6/24+14/24= 35/24 = 1 11/24

б) 5/6-3/8+1/12 = 20/24-9/24+2/24 = 13/24

в) 3/7+11/14-2/21 = 18/42+33/42-4/42 = 47/42 = 1 5/42