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Bonsoir
7x +7 y = 420
7x - 7y = 112
donc
x+y = 60
x -y = 16
x = 60 - y
60 -y - y = 16
60 -2y = 16
-2y = 16 -60
-2y = --44
-y = -22 donc y = 22
x + 22 = 60
x = 60-22
x = 38
7x +7 y = 420
7x - 7y = 112
donc
x+y = 60
x -y = 16
x = 60 - y
60 -y - y = 16
60 -2y = 16
-2y = 16 -60
-2y = --44
-y = -22 donc y = 22
x + 22 = 60
x = 60-22
x = 38