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How long does it take the giraffe to weigh 85 kg?

  1. Mar 30, 2008 #1
    1. The problem statement, all variables and given/known data
    The weight, W kg, of a giraffe over its first two years of life, is given by the equation
    W=(t^2)/4-t+68
    where t is the time in months since the giraffe was born.
    How long does it take the giraffe to weigh 85 kg?

    2. Relevant equations



    3. The attempt at a solution


    I know i need to rearage the equation but i cant remember how so any help is appreciated...
     
    Last edited by a moderator: Jul 18, 2014
  2. jcsd
  3. Mar 30, 2008 #2
    Which letter represents the weight? Put 85 there and solve it.
     
  4. Mar 30, 2008 #3
    Yeah I understand that but how would I solve it?

    its the (t^2)/4 bit i don't understand how to solve...
     
  5. Mar 30, 2008 #4
    Let me see if this is the thing you have:
    [tex]\frac{t^2}{4}-t+68=85[/tex]
    That's a quadratic equation. Before you can solve, what does the equation have to look like?
    Then, since fractions give me a headache, like like to ditch them. Can you think of a method of getting rid of them?
     
  6. Mar 30, 2008 #5
    multiply 4 to both sides?
     
  7. Mar 30, 2008 #6
    Yes please.
     
  8. Mar 30, 2008 #7
    So it would be:
    t^2-t+68=340?
     
  9. Mar 30, 2008 #8
    so you have:
    [tex]4(\frac{t^2}{4}-t+68=85)[/tex]
    make sure that you are careful about your distributive property.
    EVERYTHING gets multiplied by 4.
     
  10. Mar 30, 2008 #9
    but doesn't the t^2 already have the /4 so wouldn't it be:

    t^2+4(-t+68=85)?
     
  11. Mar 30, 2008 #10
    yes
    [tex]4(\frac{t^2}{4}-t+68=85)[/tex]
    [tex]t^2-(4)t+(4)68=(4)85[/tex]
    then what?
     
  12. Mar 30, 2008 #11
    expand.

    So it is t^2-4t+272=340

    Is that right?
     
  13. Mar 30, 2008 #12
    yes.
    In order to solve ANY equation, what does it have to be set equal to?
    There's ONE MORE STEP before you can solve it.
     
  14. Mar 30, 2008 #13
    I can't remember... :(
     
  15. Mar 30, 2008 #14
    Look for an example in your book.
     
  16. Mar 30, 2008 #15
    It has to be in the form ax=b

    thats what my textbook says.... is that what you meant?
     
  17. Mar 30, 2008 #16
    You are looking for the ZEROS
    What would you set the equation equal to to get the ZEROS?
     
  18. Mar 30, 2008 #17
    i'm lost....
     
  19. Mar 30, 2008 #18
    In order to solve any equation, it must be set equal to ZERO!
    HOORAY!
    How do you manipulate your equation to get it equal to zero?
     
  20. Mar 30, 2008 #19
    I see. take 340 from both sides so the equation is:t^2-4t-68=0 is that correct?
     
  21. Mar 30, 2008 #20
    Sit down and read the chapter ... do every single example problem!!! It will serve you better than trying to solve this problem.
     
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