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chrysanthemum
Why solving systems of equations by substitution, you substitute the expression for one variable for that variable in the other equation.

8.
y = -1/4x
x + 2y = 4

Substitute -1/4x for y in x + 2y = 4 and solve:

x + 2(-1/4x) = 4
      x - 1/2x = 4
             x/2 = 4
                x = 8

Now that you have x, you can substitute it into one of the original equations to find y:

x + 2y = 4
8 + 2y = 4
      2y = -4
        y = -2

The last step is to check your work by substituting back into both original equations:

 y = -1/4x
-2 = -1/4(8)
-2 = -2

    x + 2y = 4
8 + 2(-2) = 4
      8 - 4 = 4
           4 = 4

Answer:
x = 8
y = -2

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