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Math Help - system of equations alg II

  1. #1
    Junior Member eah1010's Avatar
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    Exclamation system of equations alg II

    Once again, easy but i can't remember...sorry for another question but i think its my last one...

    Solve the system of equations and check your answer(s). You may use any method to solve.

    2x^2 - y = 8
    7x + y = -4


    I tried it this way:

    y = -7x - 4

    2x^2 - (-7x - 4) = 8

    2x^2 + 7x + 4 = 8

    2x^2 + 7x - 4 = 0

    (2x -1) (x + 4) = 0

    x = 1/2, -4

    Now, when I plug in both values for x (because there were 2 values) seperate times i get values for y that don't check out when I check both values for x and y. HELP!!! Thank you so much!
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  2. #2
    Moo
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    Hello,

    Quote Originally Posted by eah1010 View Post
    Once again, easy but i can't remember...sorry for another question but i think its my last one...

    Solve the system of equations and check your answer(s). You may use any method to solve.

    2x^2 - y = 8
    7x + y = -4


    I tried it this way:

    y = -7x - 4

    2x^2 - (-7x - 4) = 8

    2x^2 + 7x + 4 = 8

    2x^2 + 7x - 4 = 0

    (2x -1) (x + 4) = 0

    x = 1/2, -4

    Now, when I plug in both values for x (because there were 2 values) seperate times i get values for y that don't check out when I check both values for x and y. HELP!!! Thank you so much!
    This is normal, because there are a couple of solutions !

    For the value x=1/2, y=-15/2.

    For the value x=-4, y=24.

    So the set of solutions is :

    S=\left\{\left(\frac 12, -\frac{15}{2}\right) ~,~ (-4,24)\right\}

    Is it better this way ?
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  3. #3
    Junior Member eah1010's Avatar
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    Well, it's probably my math because I am extremely bad at making little mistakes, but when i plug 1/2 into the 1st equation i got y=4.5 and when I plug in -4 as x i get y=36. Im just confused on the whole thing now! Haha! any help?
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  4. #4
    Moo
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    Quote Originally Posted by eah1010 View Post
    Well, it's probably my math because I am extremely bad at making little mistakes, but when i plug 1/2 into the 1st equation i got y=4.5 and when I plug in -4 as x i get y=36. Im just confused on the whole thing now! Haha! any help?
    y=-7x-4

    If x=1/2
    y=-7/2-4=-15/2=-7.5

    If x=-4
    y=-7*(-4)-4=28-4=24

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