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Quadratic expression help

  1. Jul 25, 2014 #1
    O2FMJiw.png



    2x^2+9x = -5

    Solved, you can banish this thread now mods.







    2x^2+9x = -5



    With the given answers, nothing I can come up with matches it. I can't solve it like a normal quadratic, as it gives me no answer that matches those. And I can't factor it. Nothing that adds up to 9, that when you also multiply them, you get -5.

    Divide off the 2...?
    Divide off the 9.
    Now Ive got X + X or X^2 on the left side, and thats already not a answer match, cant solve it for X.
    Cant factor it.

    Square rooting it just gives The suare root of 5 divided by something, thats not a answer. So Im done, not including I feel pretty dumb not being able to solve this basic math but Im doing Thermodynamics at the moment. I cant remember what to do with this...

    Only thing I heard of was to get it in standard form or something. Nothing I learned. Its this little problem that I cant figure out...

    Could someone explain how to solve this, according to the answers given, and why it works? I asked others, even other friends but even they don't know, even asked a Cop at the store the other day and he didn't know. And I cant find any questions exactly like this online, is this question that tedious? Its not even difficult I bet.

    Thanks
     
    Last edited by a moderator: Jul 25, 2014
  2. jcsd
  3. Jul 25, 2014 #2

    Nathanael

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    Homework Helper

    The "standard form" you spoke of is [itex]ax^2+bx+c=0[/itex] (you just get all the terms on to one side)

    The reason you would want to do this is so you can use the quadratic equation. (If you don't remember the quadratic equation, it should be very easy to look up.)

    As to why the quadratic equation works? It is just a generalized solution for an equation of this form (I don't want to derive it for you, but, if you're curious, that should also be easy to look up)



    P.S.
    In the future, the math-homework section of these forums will probably give you more help on purely math problems.
     
  4. Jul 25, 2014 #3
    Ooooooooooh hahaha, I feel pretty dumb I forgot all about the quadratic formula. I just plugged all this right in after putting it in standard form and that did the trick.

    Im all good now. Im going to edit my post away so I dont embarass myself even further, thanks Nathan. : D
     
  5. Jul 25, 2014 #4

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    It is against the PF Rules to edit your OP after you have solved the problem. Please leave it as-is in the future.
     
  6. Jul 25, 2014 #5

    Evo

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    Staff: Mentor

    I restored the post, helpers please remember to first quote the OP before responding, thanks!
     
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