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Mathematics · unit version 1

Precalculus: polar curves and signed-radius evidence

Construct complete noncircle polar curves, restrict their traces, and distinguish signed radius from distance and interpolation evidence without calculus.

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Readiness and preparation

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Where does a negative polar radius place a point?

Curve construction must preserve the sign before converting to Cartesian coordinates.

Compare reasoning and find a refresher

The point lies opposite the terminal ray of its angle. Use x=r cos θ and y=r sin θ; the distance from the origin is |r|. For example, r=−1 at θ=π gives (1,0), at distance 1.

Review signed polar coordinates

Is change in signed radius the same as change in distance?

The two quantities can move in opposite directions when the radius is negative.

Compare reasoning and find a refresher

No. As r decreases from −1 to −2, distance increases from 1 to 2. Keep the angle interval and the scalar quantity explicit; neither change alone measures Cartesian travel length.

Review polar graphs and radial change

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Polar curve evidence: mixed practice

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Lesson sequence

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  1. Construct complete signed-radius curves

    Lesson details and objectives

    Trace noncircle polar curves with exact sign intervals and meaningful angular restrictions.

    • Construct both loops of a limacon and all petals of a rose using exact zeros, signed radii and equal-scale coordinates.
    • Select complete angular restrictions for a requested curve portion, including negative-radius direction changes.
    • Use sign, symmetry and ray arguments to justify complete curves beyond finite plotted samples.

    6 practice tasks.

  2. Separate signed radius, distance and estimates

    Lesson details and objectives

    Prove distance intervals and use a polar table without inventing between-point accuracy.

    • Find nearest/farthest points and compare signed-radius and absolute-distance trends over complete angle intervals.
    • Execute signed radial interpolation using actual angular widths and convert the estimate to distance and Cartesian coordinates.
    • Distinguish interpolation assumptions and error limits from arc length, motion or observed curve behavior.

    6 practice tasks.