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Find the slope of the tangent at the given angle theta

  1. Jan 30, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the slope of the tangent line to the give polar curve at the point specified by the value of theta
    R = 1/θ, θ = π

    2. Relevant equations
    Slope of a polar equation is (dR/dθsinθ+Rcosθ/dR/dθcosθ-Rsinθ)

    3. The attempt at a solution
    Using my calculator, I plugged π for θ, -.101 for DR/Dtheta (after differentiating R) and R = 1/π
    I got π/12 as my answer, but the correct answer is -π. Why am I off by a factor of 12?
     
  2. jcsd
  3. Jan 30, 2015 #2

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    You'll need to show the details of how you got π/12. I don't see where the factor of 12 could come from.
     
  4. Jan 30, 2015 #3

    Mark44

    Staff: Mentor

    Your formula is pretty much unreadable. I have no idea what this means.
    [QUOTE="Calpalned"

    3. The attempt at a solution
    Using my calculator, I plugged π for θ, -.101 for DR/Dtheta (after differentiating R) and R = 1/π
    I got π/12 as my answer, but the correct answer is -π. Why am I off by a factor of 12?[/QUOTE]
    In general we have x = rcos(θ) and y = rsin(θ). Differentiate both with respect to θ to get dy/dx. When I do this, I get the answer you're supposed to get.
     
  5. Jan 30, 2015 #4
    [itex]\frac{(dr/dθ)sinθ + rcosθ}{(dr/dθ)cosθ - rsinθ}
    frac{(dr/dθ)sinθ + rcosθ}{(dr/dθ)cosθ - rsinθ}
    [itex]\frac{([itex]\frac{dr}{dθ}sinθ + rcosθ}{([itex]\frac{dr}{dθ}cosθ - rsinθ}
    [itex]frac(dr)
     
  6. Jan 30, 2015 #5

    Mark44

    Staff: Mentor

    Fixed.
    What I typed was # #\frac{(dr/dθ)sinθ + rcosθ}{(dr/dθ)cosθ - rsinθ}# # (with no extra spaces between the # pairs).

    The LaTeX tags come in pairs, with [ itex ] or [ tex ] at the beginning, and [ /itex ] or [ /tex ] at the end (omit the extra spaces. I prefer to use # # at beginning and end (again without the extra spaces, for inline LaTeX, or $ $ at beginning and end for standalone stuff.

    You have r = 1/θ. When you write x = r cos(θ) and y = r sin(θ), replace r in each equation by 1/θ. Then take your derivatives. Alternatively, you could calculate dr/dθ from r = 1/θ.
     
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