Вычислите: 1) 1/4 + 3/5; 4) 3/28 + 5/14; 7) 9/25

1) 1/4 + 3/5 = (1*5 + 3*4)/20 = (5+12)/20=17/20;
2) 9/11 - 2/5 = (9*5-2*11)/55 = (45-22)/55 = 23/55;
3) 13/16 - 9/32 = (13*2 - 9*1)/32 = (26-9)/32 = 17/32;
4) 3/28 + 5/14 = (3*1+5*2)/28 = (3+10)/28 = 13/28;
5) 14/15 - 7/10 = (14*2 - 7*3)/30 = (28-21)/30 = 7/30;
6) 3/8 + 1/6 = (3*3 + 1*4)/24 = (9+4)/24 = 13/24;
7) 9/25 - 7/20 = (9*4-7*5)/100 = (36-35)/100 = 1/100;
8) 37/42 - 17/24 = (37*4-17*7)/168 = (148-119)/168 = 29/168;
9) 11/24 - 13/16 = (11*2-3*3)/48 = (22-9)/48 = 13/48;;
10) 9/16 + 7/24 - 3/8 = (9*3+7*2-3*6)/48 = (27+14-18)/48 = 23/48;
11) 1/3 - 1/6 + 1/4 = (1*4-1*2+1*3)/12 = (4-2+3)/12 = 5/12;
12) 2/5 + 4/15 - 5/9 = (2*9+4*3-5*5)/45 = (18+12-25)/45 = 5/45 = 1/9.