Réponses

2014-05-29T12:16:09+02:00
5x+4y = -2
x+2y = 4     on la multiplie par -2 : -2x-4y = -8
on additionne les 2
5x+4y-2x-4y = -2-8
3x = -10
x = -10/3
-10/3+2y = 4
2y = 4+10/3
2y = 12/3+10/3
2y = 22/3
y = 11/3

20x+50y = 750
x+y = 21  soit x = 21-y
20(21-y)+50y = 750
420-20y+50y = 750
30y = 750-420
30y = 330
y = 330/30
y = 11   et x = 21-11 = 10