Вычислите : 1)3/7+4/9; 2)8/9-7/8;3)13/15-2/3;4)20/21+3/7;5) 17/18-11/12;6)7/16+1/6;7)2/9+5/6;8)10/21+9/14;9)7/9-4/15;10)9/14-3/7+15/28 11)1/6+1/4-1/8 12)13/18-29/45+8/15

Найдем значение выражений:
1) 3/7 + 4/9 = 27/63 + 28/63 = (27 + 28)/63 = 55/63;
2) 8/9 - 7/8 = 64/72 - 63/72 = (64 - 63)/72 = 1/72;
3) 13/15 - 2/3 = 13/15 - 10/15 = (13 - 10)/15 = 3/15 = 1/5;
4) 20/21+ 3/7 = 20/21 + 9/21 = (20 + 9)/21 = 29/21 = 1 8/21;
5) 17/18 - 11/12 = 34/36 - 33/36 = (34 - 33)/36 = 1/36;
6) 7/16 + 1/6 = 21/48 + 8/48 = (21 + 8)/48 = 29/48;
7) 2/9 + 5/6 = 4/18 + 15/18 = (4 + 15)/18 = 19/18 = 1 1/18;
8) 10/21 + 9/14 = 20/42 + 27/42 = (10 + 20)/ 42 = 30/42 = 5/7;
9) 7/9 - 4/15 = 35/45 - 12/45 = (35 - 12)/45 = 23/45;
10) 9/14 - 3/7 + 15/28 = 18/28 - 12/28 + 15/28 = (18 - 12 + 15)/28 = 21/28;
11) 1/6 + 1/4 - 1/8 = 4/24 + 6/24 - 3/24 = (4 +6 - 3)/24 = 7/24;
12) 13/18 - 29/45 + 8/15 = 65/90 - 58/90 + 48/90= (65 - 58 + 48)/90 = 55/90 = 11/18;