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Math Help - quadtratic formula problem

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    quadtratic formula problem

    Hello. I have a problem 80x+80=9x^2+18x It says use the quadtaric forumla to obtain x=8,10.

    I get the 8, but I can't figure out how they got 10. I keep getting 10/9. Thanks for any help!
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    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by Mathfailure View Post
    Hello. I have a problem 80x+80=9x^2+18x It says use the quadtaric forumla to obtain x=8,10.

    I get the 8, but I can't figure out how they got 10. I keep getting 10/9. Thanks for any help!
    10 is not a solution. plug it in to both sides and you can see that
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    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by Mathfailure View Post
    Hello. I have a problem 80x+80=9x^2+18x It says use the quadtaric forumla to obtain x=8,10.

    I get the 8, but I can't figure out how they got 10. I keep getting 10/9. Thanks for any help!

    Recall the quadratic formula.

    For a quadratic of the form ax^2 + bx + c = 0, the roots are given by:

    x = [-b +/- sqrt(b^2 - 4ac)]/2a

    now to do your problem:

    80x + 80 = 9x^2 + 18x
    => 9x^2 + 18x - 80x - 80 = 0 ...........moved everything to one side, hope you know how to do that
    => 9x^2 - 62x - 80 = 0
    By the quadratic formula:

    x = [62 +/- sqrt(3844 - 4(9)(-80))]/2(9)
    ..= [62 +/- sqrt(6724)]/18
    ..= [62 +/- 82]/18
    ..= (62 + 82)/18 or (62 - 82)/18
    ..= 8 or -10/9

    so your answer is almost correct, it should be a minus 10/9
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