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Thread: System of Equations

  1. #1
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    System of Equations

    Solve the following system of equation for y.

    2x - y = 13

    2x = y

    I came up with no solution for y.

    The book's answer is y = -39.

    Who's right?
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  2. #2
    No one in Particular VonNemo19's Avatar
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    It would appear that, in this case, you are right.

    Because if $\displaystyle 2x=y$ and then we substitute for $\displaystyle y$ in the othe equation we get $\displaystyle 2x-2x=13

    $ I don't think so! because $\displaystyle 2x-2x=0\neq13$

    Are you sure you copied the problem down right?
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  3. #3
    No one in Particular VonNemo19's Avatar
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    You could also substitute $\displaystyle -39$ for $\displaystyle y$wherever it occurs in both equations. You will find that
    $\displaystyle 2x-(-39)=1$
    $\displaystyle 2x+39=13$
    $\displaystyle 2x=-26$
    $\displaystyle x=-13$

    and now for the other equation

    $\displaystyle 2x=-39$
    $\displaystyle x=\frac{-39}{2}$

    and since the coordinates $\displaystyle (\frac{-39}{2},-39)\neq(-13,-39)$there is no solution.

    So, you got a bad book baby!
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  4. #4
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    yes...

    Quote Originally Posted by VonNemo19 View Post
    It would appear that, in this case, you are right.

    Because if $\displaystyle 2x=y$ and then we substitute for $\displaystyle y$ in the othe equation we get $\displaystyle 2x-2x=13

    $ I don't think so! because $\displaystyle 2x-2x=0\neq13$

    Are you sure you copied the problem down right?
    I thank you for the quick reply. Yes, the problem is copied down right. It is probably a typo.
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  5. #5
    MHF Contributor
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    ok...I see...

    Quote Originally Posted by VonNemo19 View Post
    You could also substitute $\displaystyle -39$ for $\displaystyle y$wherever it occurs in both equations. You will find that
    $\displaystyle 2x-(-39)=1$
    $\displaystyle 2x+39=13$
    $\displaystyle 2x=-26$
    $\displaystyle x=-13$

    and now for the other equation

    $\displaystyle 2x=-39$
    $\displaystyle x=\frac{-39}{2}$

    and since the coordinates $\displaystyle (\frac{-39}{2},-39)\neq(-13,-39)$there is no solution.

    So, you got a bad book baby!
    The book is Math A by Lawrence S. Leff. I have found many typos in his book. He is also the author of Math B. There are WAY TOO MANY errors in his books. If I can spot them, anyone can.
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