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Homework Help: How do I finish this polar equations problem?

  1. Sep 6, 2014 #1
    1. The problem statement, all variables and given/known data

    Find the points on the given curve where the tangent is horizontal or vertical

    2. Relevant equations

    r = 3cos(θ)

    3. The attempt at a solution

    d/dθ = -3sin(θ)

    for horizontal:

    -3sin(θ)sin(θ) + 3cos(θ)cos(θ)

    I used identity and got:

    3cos(2θ) = 0

    I got the values PI/4, 3PI/4

    How do I get the values 3/sqrt(2) and -3/sqrt(2)

    to give me points (3/sqrt(2), Pi/4) and (-3/sqrt(2), Pi/4)
  2. jcsd
  3. Sep 7, 2014 #2


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    Staff Emeritus
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    You found some values for ##\theta##. Use the equation you started with to get the corresponding values for ##r##.
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