Решите систему способом подстановки 6 х/7+5 у/21=5 9 х/4-у/12=11 7 х/3-у/9=4 8 х/5-у/3-=3.

1). {6 х / 7 + 5 у / 21 = 5;

{9 х / 4 - у / 12 = 11;

{18 x + 5 y = 105;

{27 x - y = 132; y = 27 x - 132;

18 x + 5 (27 x - 132) = 105;

18 x + 135 x - 660 = 105; 153 x = 765; x = 765 : 153;

x = 5;

y = 27 * 5 - 132; y = 135 - 132;

y = 3;

2). {7 х / 3 - у / 9 = 4;

{8 х / 5 - у / 3 = 3;

{21 x - y = 36; y = 21 x - 36;

{24 x - 5 y = 45;

24 x - 5 (21 x - 36) = 45;

24 x - 105 x + 180 = 45; 81 x = 135; x = 135 : 81;

x = 5/3;

y = 21 * 5/3 - 36; y = 35 - 36; y = - 1;

x = 1 2/3; y = - 1.