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Math Help - solve for x

  1. #1
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    solve for x

    cos2x=0

    x=pi/4 but how?
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  2. #2
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    Quote Originally Posted by tbenne3 View Post
    cos2x=0

    x=pi/4 but how?
    because \cos\left(2 \cdot \frac{\pi}{4}\right) = \cos\left(\frac{\pi}{2}\right) = 0<br />

    also note that there are more solutions ... x = \frac{3\pi}{4} for instance.
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  3. #3
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    Quote Originally Posted by tbenne3 View Post
    cos2x=0

    x=pi/4 but how?

    make 2x = \theta

    so \cos \theta = 0 \implies \theta = \frac{\pi}{2} \implies 2x = \frac{\pi}{2} \implies x = \frac{\pi}{4}
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  4. #4
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    Remember that we are talking about radians and not degrees! Review the unit circle. You will see that wherever the circle crosses the y axis, the cosine of that value is always zero. If you type in [pi]/4 into your calculator and it is set in degrees, you will be very confused.
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