Б) 1/3 + 1/7 = (1*7 + 1*3) / 21 = 10 / 21
в) 3/5 + 3/4 = (3*4 + 3*5) / 20 = 27 / 20 = 1 целая 7/20
г) 1/2 + 7/9 = (1*9 + 7*2) / 18 = 23 / 18 = 1 целая 5/18
д) 5/7 + 0 = 5/7
е) 2/3 - 2/5 = (2*5 - 2*3) / 15 = 4/15
ж) 1/2 - 1/3 = (1*3 - 1*2) / 6 = 1/6
з) 3/5 - 4/7 = (3*7 - 4*5) / 35 = 1/35