Sagot :
{3X-5Y=5
{4X+7Y=-7
{12X-20Y=20
{12X+21Y=-21
12X - 20Y - (12X+21Y) = 20 - (-21)
12X - 20Y -12X - 21Y = 41
-41Y = 41
Y = -1
3X-5Y=5
3X-5*(-1)= 5
3X + 5 = 5
3X = 0
x = 0
voilà :)
{3X - 5Y= 5
{4X + 7Y= -7
-4{3X - 5Y= 5
3{4X + 7Y= -7
+20y +21y = -20 -21
41y = -41
y = -41/41
y = -1
7{3X - 5Y = 5
5{4X + 7Y = -7
21x +20x = 35 - 35
41x = 0
x = 0/41
x = 0