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algebra

posted by on .

A. 3x + y = 8
x-y =6

B. x- 1/2y =2
y=2x-4

C. x + 2y = 9
2y = -x + 13

I am not grasping the substitution method! Please help me.

  • algebra - ,

    Pick the equation for which it is easier to "isolate" one of the variables.

    A) from the second: x = 6+y
    plug that into the first
    3x + y = 8
    3(6+y) + y = 8
    18 + 3y + y = 8
    4y = -10
    y = -5/2

    now put that back into the easier of the two
    x = 6+y
    x = 6 - 5/2 = 7/2

    b) really easy, y is already isolated

    c) notice we have 2y isolated , but 2y shows up in the first, so again, real easy

    x + 2y = 9
    x + (-x + 13) = 9
    13 = 9 ?????

    ahhh, this last pair of equation has no solution, (we got a silly statement)

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