Plotting polar equations requires the use of polar coordinates, in which points have the form , where r measures the radial distance from the pole O to a point P and measures the counterclockwise angle from the positive polar axis to the line segment OP.
When plotting a polar function, , it is often helpful to first plot the function on a rectilinear grid, treating as Cartesian coordinates, with being plotted along the horizontal axis and r being plotted along the vertical axis. From this Cartesian plot, you can transfer critical points, such as minima, maxima, roots, and endpoints to a polar grid using the coordinates , and then fill in the behavior of r between the critical values of to create the final polar graph.
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Common Figures in Polar Plots
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Circle: The equation creates a circle of radius a centered at the pole. The equation creates a circle of diameter a centered at the point . The equation creates a circle of diameter a centered at the point .
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Cardioid: The equations and create horizontal and vertical heart-like shapes called cardioids.
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Archimedean Spiral: any equation of the form creates a spiral, with the constant n determining how tightly the spiral winds around the pole. Special cases of Archimedean spirals include: Archimedes' Spiral when , Fermat's Spiral when , a hyperbolic spiral when , and a lituus when .
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Polar Rose: Equations of the form create curves which look like petaled flowers, where a represents the length of each petal and k determines the number of petals. If k is an odd integer, the rose will have k petals; if k is an even integer, the rose will have petals; and if k is rational, but not an integer, a rose-like shape may form with overlapping petals.
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Ellipse: Equations of the form , where is the eccentricity of the curve (a measure of how much a conic section deviates from being circular) and l is the semi-latus rectum (half the chord parallel to the directrix passing through a focus), create ellipses for which one focus is the pole and the other lies somewhere along the line . The special case where creates a circle of radius l.
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