Вычислите: 1) 3/7 + 4/9; 4) 20/21 + 3/7; 7) 2/9

1) 3/7 + 4/9 = (3*9 + 4*7)/63 = (27+28)/63=55/63;
2) 8/9 - 7/8 = (8*8-7*9)/72 = (64-63)/72 = 1/72;
3) 13/15 - 2/3 = (13*1 - 2*5)/15 = (13-10)/15 = 3/15 = 1/5;
4) 20/21 + 3/7 = (20*1+3*3)/21 = (20+9)/21 = 29/21;
5) 17/18 - 11/12 = (17*2 - 11*3)/36 = (34-33)/36 = 1/36;
6) 7/16 + 1/6 = (7*3 + 1*8)/48 = (21+8)/48 = 29/48;
7) 2/9 + 5/6 = (2*2+5*3)/18 = (4+15)/18 = 19/18;
8) 10/21 + 9/14 = (10*2 + 9*3)/42 = (20+27)/42 = 47/42;
9) 7/9 - 4/15 = (7*5 - 4*3)/45 = (35-12)/45 = 23/45;
10) 9/14 - 3/7 + 15/28 = (9*2 - 3*4 +15*1)/28 = (18-12+15)/28 = 21/28 = 3/4;
11) 1/6 + 1/4 - 1/8 = (1*6+1*4-1*3)/24 = (6+4-3)/24 = 7/24;
12) 13/18 - 29/45 + 8/15 = (13*5-29*2+8*6)/90 = (65-58+48)/90 = 55/90 = 11/18.