Polar Curves

AP Type Questions 9

Polar Curves for BC only.

Ideally, as with parametric and vector functions, polar curves should be introduced and covered thoroughly in a precalculus course. Questions on the BC exams have been concerned with calculus ideas. Students have not been asked to know the names of the various curves (rose, curves, limaçons, etc.). The graphs are usually given in the stem of the problem, but students should know how to graph polar curves on their calculator.

What students should know how to do

  • Find the intersection of two graphs (to use as limits of integration).
  • Find the area enclosed by a graph or graphs using the formula \displaystyle A=\tfrac{1}{2}\int_{{{\theta }_{1}}}^{{{\theta }_{2}}}{(r(}θ\displaystyle ){{)}^{2}}dθ
  • Use the formulas x(θ) = r(θ)cos(θ)  and y(θ) = r(θ)sin(θ) to
    • convert from polar to rectangular form,
    • calculate the coordinates of a point on the graph, and
    • calculate \frac{dy}{d\theta } and \frac{dx}{d\theta } (Hint: use the product rule).
    • Discuss the motion of a particle moving on the graph by discussing the meaning of \frac{dr}{d\theta } (motion towards or away from the pole), \frac{dy}{d\theta } (motion in the vertical direction) or \frac{dx}{d\theta } (motion in the horizontal direction).
    • Find the slope at a point on the graph, \frac{dy}{dx}=\frac{dy/d\theta }{dx/d\theta }.

This topic appears only occasionally on the free-response section of the exam. The most recent were 2007 and 2013. If the topic is not on the free-response then 1, or maybe 2 questions, probably finding area, can be expected on the multiple-choice section.

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