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Homework Help: Similtanious equations

  1. Mar 19, 2008 #1
    1. The problem statement, all variables and given/known data

    Three equations (as seen in the relevant equations section) have a y value of 0 when they initially begin at x=0. If x is measured in complete days, how many days will it be before these three functions once again share the y=0 value?

    2. Relevant equations

    y = sin (2piex/23)
    y = sin (2piex/28)
    y = sin (2piex/33)

    3. The attempt at a solution

    well these equations must all equal 0, therefore y = sin (2piex/23) = sin (2piex/28) = sin (2piex/33) = 0

    as to how to manipulate that to get the desired answer, iam not too sure since ive never done it with three equations before.
    i'd appreciate any help :)
  2. jcsd
  3. Mar 19, 2008 #2


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    HINT: For what values of the argument is sine zero?
  4. Mar 19, 2008 #3
    well if we are talking radians: pie and 2pie (then obviously continuing on with 3pie, 4 pie,etc)

    not quite seeing what your getting at though
    unless y = pie or something
    and thanks for replying :)
    Last edited: Mar 19, 2008
  5. Mar 19, 2008 #4


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    Correct. So from,

    [tex]\sin\left(\frac{2\pi x}{23}\right) = \sin\left(\frac{2\pi x}{28}\right) = \sin\left(\frac{2\pi x}{33}\right) = 0[/tex]

    We can deduce that,

    [tex]\frac{2\pi x }{23} = \frac{2\pi x }{28} = \frac{2\pi x }{33} = n\pi \hspace{2cm}, n\in\mathbb{Z}[/tex]

    [tex]\Rightarrow \frac{2}{23}x = \frac{2}{28}x = \frac{2}{33} x= n \hspace{2cm}, n\in\mathbb{Z}[/tex]

    Which in words means that "for which value of x makes all the quotients into integers"?
  6. Mar 20, 2008 #5
    i understand how you derive that mathematically, but cant think as how to solve for it since it says that 2x/23 = 2x/28 which, the way iam looking at it, means that 23 = 28
  7. Mar 20, 2008 #6
    What Hootenanny is saying is not really

    [tex]\frac{2\pi x }{23} = \frac{2\pi x }{28} = \frac{2\pi x }{33} = n\pi \text{ where } n\in\mathbb{Z}[/tex]


    [tex]\frac{2\pi x}{23} = n_1\pi; \frac{2\pi x}{28} = n_2\pi; \frac{2\pi x}{33} = n_3\pi \text{ where } n_1, n_2, n_3 \in \mathbb{Z}[/tex]

    The integers n1, n2, and n3 will not be equal, except at x = 0 (in which case they all equal 0), but what is important is that they are all integers, because sine of any multiple of [itex]\pi[/itex] will equal 0.

    What values of x > 0 will make n1, n2, and n3 all integers? Which is the smallest?
  8. Mar 20, 2008 #7
    how do i find that out with three equations though?
  9. Mar 20, 2008 #8
    The equations we gave you are the three equations. Each one has two variables, x and ni, so you cannot find unique answers. However, it is important to note that ni is an integer. What values of x give integer values of each ni? What is the smallest? Try to think it out logically.
  10. Mar 20, 2008 #9
    okay, so 2piex/23 = n1

    3.660563691=x for n1=1

    4.456338407=x for n2=1

    5.252113122=x for n3=1

    right, so i am to find the lowest common multiple inorder to find at what value of x is when all values of y=0

    i know how to do lowest common multiple, for the long way, but i have a feeling this will go on for a while before the three cross paths. What's the simplist way to work out the LCM in this instance?
  11. Mar 21, 2008 #10
    You are right, you do need to find the LCM. It would be difficult if there was a [itex]\pi[/itex] in there. Luckily, you wrote down the equations wrong. They are actually

    [tex]\frac{2\pi x}{23} = n_1\pi; \frac{2\pi x}{28} = n_2\pi; \frac{2\pi x}{33} = n_3\pi[/tex]​

    Look at what happens to [itex]\pi[/itex].
  12. Mar 21, 2008 #11
    right, thanks for pulling me up on that. so

    2/23 = 0.086956521 x=11.5000001
    2/28 = 0.071428571 x= 14.00000008
    2/33 = 0.06060606 x= 16.50000017

    so now iam to do the tedious job of multiplying them out until i find one where all the x's equal the same. is there any easier way of doing that? because this is going to go on for a while even now pie isnt involved.
  13. Mar 22, 2008 #12
    i get it. so x=2656.50006565

    which is the LCM of the three numbers

    right, thanks a lot Hootenanny and Tedjn :)
    and thanks for your patients
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