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Math Help - Distance Formula with given Distance but not X!

  1. #1
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    Distance Formula with given Distance but not X!stil needs help!

    okay, the question is...
    18. The distance between point A and B is 5times Squareroot 5 units. If point A is -4,-7 and point B is x,3 whats the X?

    i think ur supposed to put it in standard form but ive tried it but i got confused and didnyt know what to do next

    oh and i plugged everything into the distance formula and guessed that x is 1 and it worked but i want to why its 1 and stuffs
    Last edited by sacred3; November 5th 2008 at 09:25 PM.
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  2. #2
    MHF Contributor Mathstud28's Avatar
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    Quote Originally Posted by sacred3 View Post
    okay, the question is...
    18. The distance between point A and B is 5times Squareroot 5 units. If point A is -4,-7 and point B is x,3 whats the X?

    i think ur supposed to put it in standard form but ive tried it but i got confused and didnyt know what to do next

    oh and i plugged everything into the distance formula and guessed that x is 1 and it worked but i want to why its 1 and stuffs
    You just must solve

    5\sqrt{5}=\sqrt{(x+4)^2+100}
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  3. #3
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    how did u get x+4? i thought its -4 -x( sorry im not that good at math)
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  4. #4
    MHF Contributor Mathstud28's Avatar
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    Quote Originally Posted by sacred3 View Post
    how did u get x+4? i thought its -4 -x( sorry im not that good at math)
    No problem whatsoever! They are equivalent in this case.

    Consider that

    \left(-x-4\right)^2=\left(-(x+4)\right)^2=(-1)^2\cdot(x+4)^2=(x+4)^2
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  5. #5
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    for x+4 squared can i do x squared +16?

    cuz then i dont know what to do with
    <br />
(x+4)^2<br />
    Last edited by sacred3; November 5th 2008 at 07:09 PM.
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  6. #6
    Super Member Aryth's Avatar
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    (x+4)^2 = x(x + 4) + 4(x + 4) = x^2 + 4x + 4x + 16 = x^2 + 8x + 16

    Distribute each term separately and then add them together.
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  7. #7
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    im stuck :[
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  8. #8
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    please someone help me this is due tomorow and im REALLY STUCK!
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  9. #9
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    ive done everythng and ive got my C = -9 and i dont know what to do HELP! or jsut gimme answer and tell me why ! i need it for tommorrow!! HELP PLEASE!
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  10. #10
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    5\sqrt{5}=\sqrt{(x+4)^2+100}

    so:
     <br />
5\sqrt{5}=\sqrt{x^2+8x+16+100}=\sqrt{x^2+8x+116}<br />

    square to give:
     <br />
125=x^2+8x+116<br />
    then:
    x^2+8x-9=0
    factorise to get
    (x+9)(x-1)=0
    so the two solutions of x are x=1 and x=-9.
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