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What kind of equation is this?

  1. May 6, 2007 #1
    5t^2 - 100t = 480

    solve for t, book has it as one of the practice problems but doesn't show how to work it out.
     
  2. jcsd
  3. May 6, 2007 #2
    It's a quadratic equation.
     
  4. May 6, 2007 #3
    can you show me how to solve this? i'm getting stuck.
     
  5. May 6, 2007 #4
    What have you tried to do already? Have you tried factoring it, or tried applying the quadratic formula?
     
  6. May 6, 2007 #5
    yea i get {-2,18}. it doesn't look right?

    heres what i'm doing:

    5t^2 - 100t = 480

    so

    t^2 -20 + 96 = 0

    then

    [± 20 sqrt{400 - 4(1)(96)}] / 2(1) = t
     
    Last edited: May 6, 2007
  7. May 6, 2007 #6

    cristo

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    Your last equation is correct, however you have made an arithmetic error-- it does not give the solutions -2 and 18.
     
  8. May 6, 2007 #7
    The order in the numerator is not quite right it should be


    [20 ± sqrt{400 - 4(1)(96)}]/2(1) = t
     
  9. May 6, 2007 #8
    your both right, {-8,12} but that still doesn't seem right?
     
  10. May 6, 2007 #9
    Well you forgot to add change 480 to -480 since you flipped sides.
    So...[tex]5t^2-100t-480=0[/tex] now the solutions are -4 and 24 and when you put them in the equation you get the correct solution.

    [tex]\frac { 20 \pm \sqrt { (-20)^2-4 \cdot 1 \cdot (-96) } } {2 \cdot 1} [/tex]

    [tex]\frac { 20 \pm 28 } {2} [/tex]
     
    Last edited: May 6, 2007
  11. May 6, 2007 #10
    -4 works :)


    now if only the book would have mentioned that i needed to change the sign :/

    one more time to make sure i got it right.

    5t^2 - 100t = 480

    so

    t^2 -20 - 96 = 0

    then

    [± 20 sqrt{400 - 4(1)(-96)}] / 2(1) = t

    now the calc buttons 4*-96 = 384
    400-(-384) = 784
    sqrt786 = 28
    20-28 = -8
    -8/2 = -4

    thanks to all that helped!
     
  12. May 6, 2007 #11

    Office_Shredder

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    Gold Member

    The book shouldn't need to mention you change the sign, you should know how subtraction works by the time you deal with quadratic equations

    Keep in mind that it's 20±28, so you have a second solution also
     
  13. May 6, 2007 #12
    thanks:rolleyes:
     
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