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Thread: Trig word problem - solving a trig equation.

  1. #1
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    Trig word problem - solving a trig equation.

    I came across a word problem that I can't seem to puzzle through. I know the answer to it, but I can not seem to find out how to get to it.

    "A block is set in motion hanging from a spring and oscillates about it's resting position x=0, according to the function $\displaystyle x=\sqrt{3}sin2t+cos2t$. For what values of t is the block at it's resting position x=0."

    This is how I've worked it so far.

    $\displaystyle 0=\sqrt{3}sin2t+cos2t$

    $\displaystyle -cos2t=\sqrt{3}sin2t$

    $\displaystyle -\frac{cos2t}{sin2t}=\sqrt{3}$

    $\displaystyle -cot2t=\sqrt{3}$

    $\displaystyle tan2t=\frac{1}{\sqrt{3}}$

    $\displaystyle 2t=tan^-^1(\frac{1}{\sqrt{3}})$

    $\displaystyle 2t = 30\deg$

    $\displaystyle t = 15\deg$

    This answer, unfortunately, is not correct. I know that the correct answer is 75 degrees, but I have no clue how to get to that answer.

    Any help would be appreciated. I've been puzzling over this question for a while now.
    Last edited by mr fantastic; Mar 14th 2011 at 06:38 PM. Reason: Title.
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  2. #2
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    It's because if $\displaystyle \displaystyle -\cot{2t} = \sqrt{3}$ then $\displaystyle \displaystyle \tan{2t} = -\frac{1}{\sqrt{3}}$...
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    Quote Originally Posted by Prove It View Post
    It's because if $\displaystyle \displaystyle -\cot{2t} = \sqrt{3}$ then $\displaystyle \displaystyle \tan{2t} = -\frac{1}{\sqrt{3}}$...
    Your completely correct. Not sure why I forgot about the negative sign, as I had it on the paper I was typing from. However, it still turns out as -15 deg, which doesn't work.
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  4. #4
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    There will be a solution for $\displaystyle \displaystyle 2t$ in the second quadrant and the fourth quadrant.
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  5. #5
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    Oh, now I'm even more confused. The solution that the book gives is either $\displaystyle 75 \deg$, or $\displaystyle \frac{5\pi}{12}$, which are in quadrant one.
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    Because when you have the values of $\displaystyle \displaystyle 2t$, you need to divide them by $\displaystyle \displaystyle 2$ to get the values of $\displaystyle \displaystyle t$...
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  7. #7
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    Ah, I understand now. I think I had some faulty notions at play. I was approaching the negative degree as when the spring was below the zero, but I guess the motion is always positive around the unit circle, hence the reason I had to get rid of the negative? Still having a hard time wrapping my head around the underlying concept, but it should click in soon.

    Once again, Prove It, you have helped immensely. Thank you.
    Now when I get home I am going to post a problem that has sin as an exponent, and I haven't got a clue. Not sure why this chapter in the book is so difficult for me.
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