7/20 - (9/35 - 3/28) = 1/5
1) 9/35 - 3/28 = 36/140 - 15/140 = 21/140 = 3/20
2) 7/20 - 3/20 = 4/20 = 1/5
Пояснения:
Общий знаменатель 140
140 : 35 = 4 - доп.множ. к 9/35 = (9*4)/(35*4) = 36/140
140 : 28 = 5 - доп.множ. к 3/28 = (3*5)/(28*5) = 15/140
21/140 = 3/20 - сократили на 7
4/20 = 1/5 - сократили на 4
(21/22 - 5/11) - (22/39 - 3/13) = 1/6
1) 21/22 - 5/11 = 21/22 - 10/22 = 11/22 = 1/2
2) 22/39 - 3/13 = 22/39 - 9/39 = 13/39 = 1/3
3) 1/2 - 1/3 = 3/6 - 2/6 = 1/6
Пояснения:
5/11 = 10/22 - доп.множ. 2
3/13 = 9/39 - доп.множ. 3
11/22 = 1/2 - сократили на 11
13/39 = 1/3 - сократили на 13
1/2 = 3/6 - доп.множ. 3
1/3 = 2/6 - доп.множ. 2