Diketahui :3x + y=-1 dan x +3y=5 tentukan dengan cara substitusi

Bab Persamaan Linear
Matematika SMP Kelas VIII

3x + y = -1
y = -3x - 1

x + 3y = 5
x + 3 (-3x - 1) = 5
x - 9x - 3 = 5
-8x = 5 + 3
x = 8 : (-8)
x = -1

y = -3x - 1
   = -3 . (-1) - 1
   = 3 - 1   
   = 2

(-1, 2)

★3x + y = -1
✩x + 3y = 5
x = 5 - 3y

3(5 - 3y) + y = - 1
15 - 9y + y = - 1
15 - 8y = - 1
15 + 1 = 8y
16 = 8y
2 = y.

x + 3(2) = 5
x + 6 = 5
x = 5 - 6
x = - 1.