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Homework Help: Solve for trig equation with 2 variables

  1. Nov 26, 2012 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations
    3. The attempt at a solution

    I can solve the first part of the question. -2 ≤ k ≤ 2 because -1 ≤ sin(x) ≤ 1. How do I solve the second part of the question? Thanks.
  2. jcsd
  3. Nov 26, 2012 #2
    Forget k for a moment, and think about what values 2 sin(3x) can assume. For each such value v, you have 2 sin(3x) = v, or 2 sin(3x) + (-v) = 0. So your k values are the same as the values of -v.
  4. Nov 27, 2012 #3
    It may help to think about the graph of this function. It may even help to graph it. The 2 will increase the amplitude of the function and the 3 will scale the function. More specifically, it will "shrink" the function in the x direction (but it will not affect affect the amplitude). So from 0 to pi, the function will go completely up and down once and then be halfway done with a cycle. If you draw a horizontal line through this equation, how many solutions are there? (Hint: it depends on where the line is).
  5. Nov 28, 2012 #4
    http://www4c.wolframalpha.com/Calculate/MSP/MSP4251a45574034148cah00001164cg2h50a94c78?MSPStoreType=image/gif&s=5&w=300&h=183&cdf=RangeControl [Broken]

    So if k:
    is = -2, there will be 2 zeros
    is between (-2,0] there will be 4 zeros
    is between (0,2), there will be 2 zeros
    is = 2, there will be 1 zero

    Is that correct? Thanks.
    Last edited by a moderator: May 6, 2017
  6. Nov 28, 2012 #5
    That looks correct to me, but you should check your result by looking at a graph of the function, perhaps with k=0 if you haven't already done so.

    One other small matter, which you probably take for granted, but you should still mention if this is a homework or exam problem: what happens when k > 2 or k < -2?
    Last edited by a moderator: May 6, 2017
  7. Nov 28, 2012 #6
    Very close. However, the only time that there are 4 solutions is when k=0. For k (-2,0), there are 3 solutions
    Last edited by a moderator: May 6, 2017
  8. Nov 28, 2012 #7
    There are never only 3 solutions. For -2 < k < 0 just consider k = -1 and test x = π/18, 5π/18, 13π/18, 17π/18.
  9. Nov 28, 2012 #8
    Oops! Then what he said was correct
  10. Nov 28, 2012 #9
    You're right, I forgot to mention that. When k > 2 or k < 2, there are no solutions (because the amplitude is 2).

    Additionally, could I approach this problem algebraically (without referring to a graph)?

  11. Nov 28, 2012 #10
    Never mind. Thank you Michael & piercebeatz.
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