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Polar Coordinates

Locate points in a plane by using polar coordinates.

Defining Polar Coordinates

To find the coordinates of a point in the polar coordinate system, consider . The point \(P\) has Cartesian coordinates \((x,y).\) The line segment connecting the origin to the point \(P\) measures the distance from the origin to \(P\) and has length \(r.\) The angle between the positive \(x\)-axis and the line segment has measure \(\theta .\) This observation suggests a natural correspondence between the coordinate pair \((x,y)\) and the values \(r\) and \(\theta .\) This correspondence is the basis of the polar coordinate system. Note that every point in the Cartesian plane has two values (hence the term ordered pair) associated with it. In the polar coordinate system, each point also has two values associated with it: \(r\) and \(\theta .\)

Using right-triangle trigonometry, the following equations are true for the point \(P\text{:}\)

\[\text{cos}\ \theta =\frac{x}{r}\ \text{so}\ x=r\ \text{cos}\ \theta\]\[\text{sin}\ \theta =\frac{y}{r}\ \text{so}\ y=r\ \text{sin}\ \theta .\]

Furthermore,

\[{r}^{2}={x}^{2}+{y}^{2}\ \text{and}\ \text{tan}\ \theta =\frac{y}{x}.\]

Each point \((x,y)\) in the Cartesian coordinate system can therefore be represented as an ordered pair \((r,\theta )\) in the polar coordinate system. The first coordinate is called the radial coordinate and the second coordinate is called the angular coordinate. Every point in the plane can be represented in this form.

Note that the equation \(\text{tan}\ \theta =y\text{/}x\) has an infinite number of solutions for any ordered pair \((x,y).\) However, if we restrict the solutions to values between \(0\) and \(2\pi\) then we can assign a unique solution to the quadrant in which the original point \((x,y)\) is located. Then the corresponding value of r is positive, so \({r}^{2}={x}^{2}+{y}^{2}.\)

Every point in the plane has an infinite number of representations in polar coordinates. However, each point in the plane has only one representation in the rectangular coordinate system.

Condensed — the full section is in OpenStax Calculus Volume 3.

Polar Curves

Now that we know how to plot points in the polar coordinate system, we can discuss how to plot curves. In the rectangular coordinate system, we can graph a function \(y=f(x)\) and create a curve in the Cartesian plane. In a similar fashion, we can graph a curve that is generated by a function \(r=f(\theta ).\)

The general idea behind graphing a function in polar coordinates is the same as graphing a function in rectangular coordinates. Start with a list of values for the independent variable \((\theta\) in this case) and calculate the corresponding values of the dependent variable \(r.\) This process generates a list of ordered pairs, which can be plotted in the polar coordinate system. Finally, connect the points, and take advantage of any patterns that may appear. The function may be periodic, for example, which indicates that only a limited number of values for the independent variable are needed.

The graph in was that of a circle. The equation of the circle can be transformed into rectangular coordinates using the coordinate transformation formulas in . gives some more examples of functions for transforming from polar to rectangular coordinates.

We have now seen several examples of drawing graphs of curves defined by polar equations. A summary of some common curves is given in the tables below. In each equation, a and b are arbitrary constants.

A cardioid is a special case of a limaçon (pronounced “lee-mah-son”), in which \(a=b\) or \(a=\text{-}b.\) The rose is a very interesting curve. Notice that the graph of \(r=3\ \text{sin}\ 2\theta\) has four petals. However, the graph of \(r=3\ \text{sin}\ 3\theta\) has three petals as shown.

Suppose a curve is described in the polar coordinate system via the function \(r=f(\theta ).\) Since we have conversion formulas from polar to rectangular coordinates given by

Condensed — the full section is in OpenStax Calculus Volume 3.

Symmetry in Polar Coordinates

When studying symmetry of functions in rectangular coordinates (i.e., in the form \(y=f(x)),\) we talk about symmetry with respect to the y-axis and symmetry with respect to the origin. In particular, if \(f(\text{-}x)=f(x)\) for all \(x\) in the domain of \(f,\) then \(f\) is an even function and its graph is symmetric with respect to the y-axis. If \(f(\text{-}x)=\text{-}f(x)\) for all \(x\) in the domain of \(f,\) then \(f\) is an odd function and its graph is symmetric with respect to the origin. By determining which types of symmetry a graph exhibits, we can learn more about the shape and appearance of the graph. Symmetry can also reveal other properties of the function that generates the graph. Symmetry in polar curves works in a similar fashion.

Symmetry in Polar Curves and Equations

Consider a curve generated by the function \(r=f(\theta )\) in polar coordinates.

The curve is symmetric about the polar axis if for every point \((r,\theta )\) on the graph, the point \((r,\text{-}\theta )\) is also on the graph. Similarly, the equation \(r=f(\theta )\) is unchanged by replacing \(\theta\) with \(\text{-}\theta .\)
The curve is symmetric about the pole if for every point \((r,\theta )\) on the graph, the point \((r,\pi +\theta )\) is also on the graph. Similarly, the equation \(r=f(\theta )\) is unchanged when replacing \(r\) with \(\text{-}r,\) or \(\theta\) with \(\pi +\theta .\)
The curve is symmetric about the vertical line \(\theta =\frac{\pi }{2}\) if for every point \((r,\theta )\) on the graph, the point \((r,\pi -\theta )\) is also on the graph. Similarly, the equation \(r=f(\theta )\) is unchanged when \(\theta\) is replaced by \(\pi -\theta .\)

The following table shows examples of each type of symmetry.

Condensed — the full section is in OpenStax Calculus Volume 3.

Key Concepts

The polar coordinate system provides an alternative way to locate points in the plane.
Convert points between rectangular and polar coordinates using the formulas
\[x=r\ \text{cos}\ \theta \ \text{and}\ y=r\ \text{sin}\ \theta\]
and
\[r=\sqrt{{x}^{2}+{y}^{2}}\ \text{and}\ \text{tan}\ \theta =\frac{y}{x}.\]
To sketch a polar curve from a given polar function, make a table of values and take advantage of periodic properties.
Use the conversion formulas to convert equations between rectangular and polar coordinates.
Identify symmetry in polar curves, which can occur through the pole, the horizontal axis, or the vertical axis.

Polar Coordinates

In the following exercises, plot the point whose polar coordinates are given by first constructing the angle \(\theta\) and then marking off the distance r along the ray.

For the following exercises, consider the polar graph below. Give two sets of polar coordinates for each point.

For the following exercises, the rectangular coordinates of a point are given. Find two sets of polar coordinates for the point in \((0,2\pi ].\) Round to three decimal places.

For the following exercises, find rectangular coordinates for the given point in polar coordinates.

For the following exercises, determine whether the graphs of the polar equation are symmetric with respect to the \(x\)-axis, the \(y\)-axis, or the origin.

For the following exercises, describe the graph of each polar equation. Confirm each description by converting into a rectangular equation.

For the following exercises, convert the rectangular equation to polar form and sketch its graph.

Condensed — the full section is in OpenStax Calculus Volume 3.

Defining Polar Coordinates

Using right-triangle trigonometry, the following equations are true for the point \(P\text{:}\)

\[\text{cos}\ \theta =\frac{x}{r}\ \text{so}\ x=r\ \text{cos}\ \theta\]\[\text{sin}\ \theta =\frac{y}{r}\ \text{so}\ y=r\ \text{sin}\ \theta .\]

Furthermore,

\[{r}^{2}={x}^{2}+{y}^{2}\ \text{and}\ \text{tan}\ \theta =\frac{y}{x}.\]

Every point in the plane has an infinite number of representations in polar coordinates. However, each point in the plane has only one representation in the rectangular coordinate system.

Condensed — the full section is in OpenStax Calculus Volume 2.

Polar Curves

Suppose a curve is described in the polar coordinate system via the function \(r=f(\theta ).\) Since we have conversion formulas from polar to rectangular coordinates given by

Condensed — the full section is in OpenStax Calculus Volume 2.

Symmetry in Polar Coordinates

Symmetry in Polar Curves and Equations

Consider a curve generated by the function \(r=f(\theta )\) in polar coordinates.

The curve is symmetric about the polar axis if for every point \((r,\theta )\) on the graph, the point \((r,\text{-}\theta )\) is also on the graph. Similarly, the equation \(r=f(\theta )\) is unchanged by replacing \(\theta\) with \(\text{-}\theta .\)
The curve is symmetric about the pole if for every point \((r,\theta )\) on the graph, the point \((r,\pi +\theta )\) is also on the graph. Similarly, the equation \(r=f(\theta )\) is unchanged when replacing \(r\) with \(\text{-}r,\) or \(\theta\) with \(\pi +\theta .\)
The curve is symmetric about the vertical line \(\theta =\frac{\pi }{2}\) if for every point \((r,\theta )\) on the graph, the point \((r,\pi -\theta )\) is also on the graph. Similarly, the equation \(r=f(\theta )\) is unchanged when \(\theta\) is replaced by \(\pi -\theta .\)

The following table shows examples of each type of symmetry.

Condensed — the full section is in OpenStax Calculus Volume 2.

Key Concepts

The polar coordinate system provides an alternative way to locate points in the plane.
Convert points between rectangular and polar coordinates using the formulas
\[x=r\ \text{cos}\ \theta \ \text{and}\ y=r\ \text{sin}\ \theta\]
and
\[r=\sqrt{{x}^{2}+{y}^{2}}\ \text{and}\ \text{tan}\ \theta =\frac{y}{x}.\]
To sketch a polar curve from a given polar function, make a table of values and take advantage of periodic properties.
Use the conversion formulas to convert equations between rectangular and polar coordinates.
Identify symmetry in polar curves, which can occur through the pole, the horizontal axis, or the vertical axis.

Polar Coordinates

In the following exercises, plot the point whose polar coordinates are given by first constructing the angle \(\theta\) and then marking off the distance r along the ray.

For the following exercises, consider the polar graph below. Give two sets of polar coordinates for each point.

For the following exercises, the rectangular coordinates of a point are given. Find two sets of polar coordinates for the point in \((0,2\pi ].\) Round to three decimal places.

For the following exercises, find rectangular coordinates for the given point in polar coordinates.

For the following exercises, determine whether the graphs of the polar equation are symmetric with respect to the \(x\)-axis, the \(y\)-axis, or the origin.

For the following exercises, describe the graph of each polar equation. Confirm each description by converting into a rectangular equation.

For the following exercises, convert the rectangular equation to polar form and sketch its graph.

Condensed — the full section is in OpenStax Calculus Volume 2.

Practice (40)

Try each one on paper first. Reveal the answer to check; verified ones can be opened in the solver for every step.

Convert each of the following points into polar coordinates.
1. \((1,1)\)
2. \((-3,4)\)
3. \((0,3)\)
4. \((5\sqrt{3},-5)\)
Convert each of the following points into rectangular coordinates.
1. \((3,\pi \text{/}3)\)
2. \((2,3\pi \text{/}2)\)
3. \((6,-5\pi \text{/}6)\)
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1. Use \(x=1\) and \(y=1\) in :
  \[\begin{array}{lllllllllllllllllll}\begin{array}{lll}{r}^{2} & = & {x}^{2}+{y}^{2} \\ & = & {1}^{2}+{1}^{2} \\ r & = & \sqrt{2}\end{array} & & & \text{and} & & & \begin{array}{lll}\text{tan}\ \theta & = & \frac{y}{x} \\ & = & \frac{1}{1}=1 \\ \theta & = & \frac{\pi }{4}.\end{array}\end{array}\]
  Therefore this point can be represented as \((\sqrt{2},\frac{\pi }{4})\) in polar coordinates.
2. Use \(x=-3\) and \(y=4\) in :
  \[\begin{array}{lllllllllllllllllllll}\begin{array}{lll}{r}^{2} & = & {x}^{2}+{y}^{2} \\ & = & {(-3)}^{2}+{(4)}^{2} \\ r & = & 5\end{array} & & & \text{and} & & & \begin{array}{lll}\text{tan}\ \theta & = & \frac{y}{x} \\ & = & -\frac{4}{3} \\ \theta & = & \pi -\text{arctan}(\frac{4}{3}) \\ & \approx & 2.21.\end{array}\end{array}\]
  The point \((-3,4)\) lies in Quadrant \(\text{II}\). Subtract the value of the reference angle, \(\text{arctan}\left(\frac{4}{3}\right)\), from \(\pi\) to find the radian measure of \(\theta\).
  Therefore this point can be represented as \((5,2.21)\) in polar coordinates.
3. Use \(x=0\) and \(y=3\) in :
  \[\begin{array}{lllllllllllllllllll}\begin{array}{lll}{r}^{2} & = & {x}^{2}+{y}^{2} \\ & = & {(3)}^{2}+{(0)}^{2} \\ & = & 9+0 \\ r & = & 3\end{array} & & & \text{and} & & & \begin{array}{lll}\text{tan}\ \theta & = & \frac{y}{x} \\ & = & \frac{3}{0}.\end{array}\end{array}\]
  Direct application of the second equation leads to division by zero. Graphing the point \((0,3)\) on the rectangular coordinate system reveals that the point is located on the positive y-axis. The angle between the positive x-axis and the positive y-axis is \(\frac{\pi }{2}.\) Therefore this point can be represented as \((3,\frac{\pi }{2})\) in polar coordinates.
4. Use \(x=5\sqrt{3}\) and \(y=-5\) in :
  \[\begin{array}{lllllllllllllllllllll}\begin{array}{lll}{r}^{2} & = & {x}^{2}+{y}^{2} \\ & = & {(5\sqrt{3})}^{2}+{(-5)}^{2} \\ & = & 75+25 \\ r & = & 10\end{array} & & & \text{and} & & & \begin{array}{lll}\text{tan}\ \theta & = & \frac{y}{x} \\ & = & \frac{-5}{5\sqrt{3}}=-\frac{\sqrt{3}}{3} \\ \theta & = & -\frac{\pi }{6}.\end{array}\end{array}\]
  Therefore this point can be represented as \((10,-\frac{\pi }{6})\) in polar coordinates.
5. Use \(r=3\) and \(\theta =\frac{\pi }{3}\) in :
  \[\begin{array}{lllllllllllllllllll}\begin{array}{lll}x & = & r\ \text{cos}\ \theta \\ & = & 3\ \text{cos}(\frac{\pi }{3}) \\ & = & 3(\frac{1}{2})=\frac{3}{2}\end{array} & & & \text{and} & & & \begin{array}{lll}y & = & r\ \text{sin}\ \theta \\ & = & 3\ \text{sin}(\frac{\pi }{3}) \\ & = & 3(\frac{\sqrt{3}}{2})=\frac{3\sqrt{3}}{2}.\end{array}\end{array}\]
  Therefore this point can be represented as \((\frac{3}{2},\ \frac{3\sqrt{3}}{2})\) in rectangular coordinates.
6. Use \(r=2\) and \(\theta =\frac{3\pi }{2}\) in :
  \[\begin{array}{lllllllllllllllllll}\begin{array}{lll}x & = & r\ \text{cos}\ \theta \\ & = & 2\ \text{cos}(\frac{3\pi }{2}) \\ & = & 2(0)=0\end{array} & & & \text{and} & & & \begin{array}{lll}y & = & r\ \text{sin}\ \theta \\ & = & 2\ \text{sin}(\frac{3\pi }{2}) \\ & = & 2(-1)=-2.\end{array}\end{array}\]
  Therefore this point can be represented as \((0,-2)\) in rectangular coordinates.
7. Use \(r=6\) and \(\theta =-\frac{5\pi }{6}\) in :
  \[\begin{array}{lllllllllllllllllllllll}\begin{array}{lll}x & = & r\ \text{cos}\ \theta \\ & = & 6\ \text{cos}(-\frac{5\pi }{6}) \\ & = & 6(-\frac{\sqrt{3}}{2}) \\ & = & -3\sqrt{3}\end{array} & & & \text{and} & & & \begin{array}{lll}y & = & r\ \text{sin}\ \theta \\ & = & 6\ \text{sin}(-\frac{5\pi }{6}) \\ & = & 6(-\frac{1}{2}) \\ & = & -3.\end{array}\end{array}\]
  Therefore this point can be represented as \((-3\sqrt{3},-3)\) in rectangular coordinates.
Convert \((-8,-8)\) into polar coordinates and \((4,\frac{2\pi }{3})\) into rectangular coordinates.

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\((8\sqrt{2},\frac{5\pi }{4})\) and \((-2,2\sqrt{3})\)
Plot each of the following points on the polar plane.
1. \((2,\frac{\pi }{4})\)
2. \((-3,\frac{2\pi }{3})\)
3. \((4,\frac{5\pi }{4})\)
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The three points are plotted in the following figure.
Plot \((4,\frac{5\pi }{3})\) and \((-3,-\frac{7\pi }{2})\) on the polar plane.

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Graph the curve defined by the function \(r=4\ \text{sin}\ \theta .\) Identify the curve and rewrite the equation in rectangular coordinates.

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Because the function is a multiple of a sine function, it is periodic with period \(2\pi ,\) so use values for \(\theta\) between 0 and \(2\pi .\) The result of steps 1–3 appear in the following table. shows the graph based on this table.

\(\theta\)	\(r=4\ \text{sin}\ \theta\)		\(\theta\)	\(r=4\ \text{sin}\ \theta\)
0	0		\(\pi\)	0
\(\frac{\pi }{6}\)	\(2\)	\(\frac{7\pi }{6}\)	\(-2\)
\(\frac{\pi }{4}\)	\(2\sqrt{2}\approx 2.8\)	\(\frac{5\pi }{4}\)	\(-2\sqrt{2}\approx -2.8\)
\(\frac{\pi }{3}\)	\(2\sqrt{3}\approx 3.4\)	\(\frac{4\pi }{3}\)	\(-2\sqrt{3}\approx -3.4\)
\(\frac{\pi }{2}\)	\(4\)	\(\frac{3\pi }{2}\)	\(-4\)
\(\frac{2\pi }{3}\)	\(2\sqrt{3}\approx 3.4\)	\(\frac{5\pi }{3}\)	\(-2\sqrt{3}\approx -3.4\)
\(\frac{3\pi }{4}\)	\(2\sqrt{2}\approx 2.8\)	\(\frac{7\pi }{4}\)	\(-2\sqrt{2}\approx -2.8\)
\(\frac{5\pi }{6}\)	\(2\)	\(\frac{11\pi }{6}\)	\(-2\)
		\(2\pi\)	0

This is the graph of a circle. The equation \(r=4\ \text{sin}\ \theta\) can be converted into rectangular coordinates by first multiplying both sides by \(r.\) This gives the equation \({r}^{2}=4r\ \text{sin}\ \theta .\) Next use the facts that \({r}^{2}={x}^{2}+{y}^{2}\) and \(y=r\ \text{sin}\ \theta .\) This gives \({x}^{2}+{y}^{2}=4y.\) To put this equation into standard form, subtract \(4y\) from both sides of the equation and complete the square:

\[\begin{array}{lll}{x}^{2}+{y}^{2}-4y & = & 0 \\ {x}^{2}+({y}^{2}-4y) & = & 0 \\ {x}^{2}+({y}^{2}-4y+4) & = & 0+4 \\ {x}^{2}+{(y-2)}^{2} & = & 4.\end{array}\]

This is the equation of a circle with radius 2 and center \((0,2)\) in the rectangular coordinate system.

Create a graph of the curve defined by the function \(r=4+4\ \text{cos}\ \theta .\)

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The name of this shape is a cardioid, which we will study further later in this section.
Rewrite each of the following equations in rectangular coordinates and identify the graph.
1. \(\theta =\frac{\pi }{3}\)
2. \(r=3\)
3. \(r=6\ \text{cos}\ \theta -8\ \text{sin}\ \theta\)
@ action
1. Take the tangent of both sides. This gives \(\text{tan}\ \theta =\text{tan}(\pi \text{/}3)=\sqrt{3}.\) Since \(\text{tan}\ \theta =y\text{/}x\) we can replace the left-hand side of this equation by \(y\text{/}x.\) This gives \(y\text{/}x=\sqrt{3},\) which can be rewritten as \(y=x\sqrt{3}.\) This is the equation of a straight line passing through the origin with slope \(\sqrt{3}.\) In general, any polar equation of the form \(\theta =K\) represents a straight line through the pole with slope equal to \(\text{tan}\ K.\)
2. First, square both sides of the equation. This gives \({r}^{2}=9.\) Next replace \({r}^{2}\) with \({x}^{2}+{y}^{2}.\) This gives the equation \({x}^{2}+{y}^{2}=9,\) which is the equation of a circle centered at the origin with radius 3. In general, any polar equation of the form \(r=k\) where k is a positive constant represents a circle of radius k centered at the origin. (Note: when squaring both sides of an equation it is possible to introduce new points unintentionally. This should always be taken into consideration. However, in this case we do not introduce new points. For example, \((-3,\frac{\pi }{3})\) is the same point as \((3,\frac{4\pi }{3}).)\)
3. Multiply both sides of the equation by \(r.\) This leads to \({r}^{2}=6r\ \text{cos}\ \theta -8r\ \text{sin}\ \theta .\) Next use the formulas
  \[{r}^{2}={x}^{2}+{y}^{2},\ x=r\ \text{cos}\ \theta ,\ y=r\ \text{sin}\ \theta .\]
  This gives
  \[\begin{array}{lll}{r}^{2} & = & 6(r\ \text{cos}\ \theta )-8(r\ \text{sin}\ \theta ) \\ {x}^{2}+{y}^{2} & = & 6x-8y.\end{array}\]
  To put this equation into standard form, first move the variables from the right-hand side of the equation to the left-hand side, then complete the square.
  \[\begin{array}{lll}{x}^{2}+{y}^{2} & = & 6x-8y \\ {x}^{2}-6x+{y}^{2}+8y & = & 0 \\ ({x}^{2}-6x)+({y}^{2}+8y) & = & 0 \\ ({x}^{2}-6x+9)+({y}^{2}+8y+16) & = & 9+16 \\ {(x-3)}^{2}+{(y+4)}^{2} & = & 25.\end{array}\]
  This is the equation of a circle with center at \((3,-4)\) and radius 5. Notice that the circle passes through the origin since the center is 5 units away.
Rewrite the equation \(r=\text{sec}\ \theta \ \text{tan}\ \theta\) in rectangular coordinates and identify its graph.

@ action

\(y={x}^{2},\) which is the equation of a parabola opening upward.
Recall the chambered nautilus introduced in the chapter opener. This creature displays a spiral when half the outer shell is cut away. It is possible to describe a spiral using rectangular coordinates. shows a spiral in rectangular coordinates. How can we describe this curve mathematically?

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As the point P travels around the spiral in a counterclockwise direction, its distance d from the origin increases. Assume that the distance d is a constant multiple k of the angle \(\theta\) that the line segment OP makes with the positive r-axis. Therefore \(d(P,O)=k\theta ,\) where \(O\) is the origin. Now use the distance formula and some trigonometry:
\[\begin{array}{lll}d(P,O) & = & k\theta \\ \sqrt{{(x-0)}^{2}+{(y-0)}^{2}} & = & k\ \text{arctan}(\frac{y}{x}) \\ \sqrt{{x}^{2}+{y}^{2}} & = & k\ \text{arctan}(\frac{y}{x}) \\ \text{arctan}(\frac{y}{x}) & = & \frac{\sqrt{{x}^{2}+{y}^{2}}}{k} \\ y & = & x\ \text{tan}(\frac{\sqrt{{x}^{2}+{y}^{2}}}{k}).\end{array}\]
Although this equation describes the spiral, it is not possible to solve it directly for either x or y. However, if we use polar coordinates, the equation becomes much simpler. In particular, \(d(P,O)=r,\) and \(\theta\) is the second coordinate. Therefore the equation for the spiral becomes \(r=k\theta .\) Note that when \(\theta =0\) we also have \(r=0,\) so the spiral emanates from the origin. We can remove this restriction by adding a constant to the equation. Then the equation for the spiral becomes \(r=a+k\theta\) for arbitrary constants \(a\) and \(k.\) This is referred to as an Archimedean spiral, after the Greek mathematician Archimedes.
Another type of spiral is the logarithmic spiral, described by the function \(r=a\cdot {b}^{\theta }.\) A graph of the function \(r=1.2({1.25}^{\theta })\) is given in . This spiral describes the shell shape of the chambered nautilus.
Find the symmetry of the rose defined by the equation \(r=3\ \text{sin}(2\theta )\) and create a graph.
@ action
Suppose the point \((r,\theta )\) is on the graph of \(r=3\ \text{sin}(2\theta ).\)
1. To test for symmetry about the polar axis, first try replacing \(\theta\) with \(\text{-}\theta .\) This gives \(r=3\ \text{sin}(2(\text{-}\theta ))=-3\ \text{sin}(2\theta ).\) Since this changes the original equation, this test is not satisfied. However, returning to the original equation and replacing \(r\) with \(\text{-}r\) and \(\theta\) with \(\pi -\theta\) yields
  \[\begin{array}{l} \\ \text{-}r=3\ \text{sin}(2(\pi -\theta )) \\ \text{-}r=3\ \text{sin}(2\pi -2\theta ) \\ \text{-}r=3\ \text{sin}(-2\theta ) \\ \text{-}r=-3\ \text{sin}\ 2\theta .\end{array}\]
  Multiplying both sides of this equation by \(-1\) gives \(r=3\ \text{sin}\ 2\theta ,\) which is the original equation. This demonstrates that the graph is symmetric with respect to the polar axis.
2. To test for symmetry with respect to the pole, first replace \(r\) with \(\text{-}r,\) which yields \(\text{-}r=3\ \text{sin}(2\theta ).\) Multiplying both sides by −1 gives \(r=-3\ \text{sin}(2\theta ),\) which does not agree with the original equation. Therefore the equation does not pass the test for this symmetry. However, returning to the original equation and replacing \(\theta\) with \(\theta +\pi\) gives
  \[\begin{array}{ll}r & =3\ \text{sin}(2(\theta +\pi )) \\ & =3\ \text{sin}(2\theta +2\pi ) \\ & =3(\text{sin}\ 2\theta \ \text{cos}\ 2\pi +\text{cos}\ 2\theta \ \text{sin}\ 2\pi ) \\ & =3\ \text{sin}\ 2\theta .\end{array}\]
  Since this agrees with the original equation, the graph is symmetric about the pole.
3. To test for symmetry with respect to the vertical line \(\theta =\frac{\pi }{2},\) first replace both \(r\) with \(\text{-}r\) and \(\theta\) with \(\text{-}\theta .\)
  \[\begin{array}{l} \\ \text{-}r=3\ \text{sin}(2(\text{-}\theta )) \\ \text{-}r=3\ \text{sin}(-2\theta ) \\ \text{-}r=-3\ \text{sin}\ 2\theta .\end{array}\]
  Multiplying both sides of this equation by \(-1\) gives \(r=3\ \text{sin}\ 2\theta ,\) which is the original equation. Therefore the graph is symmetric about the vertical line \(\theta =\frac{\pi }{2}.\)
This graph has symmetry with respect to the polar axis, the origin, and the vertical line going through the pole. To graph the function, tabulate values of \(\theta\) between 0 and \(\pi \text{/}2\) and then reflect the resulting graph.
\(\theta\) \(r\)
\(0\) \(0\)
\(\frac{\pi }{6}\) \(\frac{3\sqrt{3}}{2}\approx 2.6\)
\(\frac{\pi }{4}\) \(3\)
\(\frac{\pi }{3}\) \(\frac{3\sqrt{3}}{2}\approx 2.6\)
\(\frac{\pi }{2}\) \(0\)
This gives one petal of the rose, as shown in the following graph.
Reflecting this image into the other three quadrants gives the entire graph as shown.
Determine the symmetry of the graph determined by the equation \(r=2\ \text{cos}(3\theta )\) and create a graph.

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Symmetric with respect to the polar axis.
\((3,\frac{\pi }{6})\)

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\((-2,\frac{5\pi }{3})\)
\((0,\frac{7\pi }{6})\)

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\((-4,\frac{3\pi }{4})\)
\((1,\frac{\pi }{4})\)

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\((2,\frac{5\pi }{6})\)
\((1,\frac{\pi }{2})\)

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Coordinates of point A.
Coordinates of point B.

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\(B\begin{array}{ll}(3,\frac{\text{-}\pi }{3}) & B(-3,\frac{2\pi }{3})\end{array}\)
Coordinates of point C.
Coordinates of point D.

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\(D(5,\frac{7\pi }{6})D(-5,\frac{\pi }{6})\)
\((8,\ 15)\)
\((-6,\ 8)\)

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\((10,2.214)(-10,5.356)\)
\((3,\text{-}\sqrt{3})\)

@ action

\((2\sqrt{3},\frac{11\pi }{6})(-2\sqrt{3},\frac{5\pi }{6})\)
\((2,\frac{5\pi }{4})\)
\((-2,\frac{\pi }{6})\)

@ action

\((\begin{array}{ll}\text{-}\sqrt{3}, & -1\end{array})\)
\((5,\frac{\pi }{3})\)
\((1,\frac{7\pi }{6})\)

@ action

\((\begin{array}{ll}-\frac{\sqrt{3}}{2}, & \frac{-1}{2}\end{array})\)
\((-3,\frac{3\pi }{4})\)
\((0,\frac{\pi }{2})\)

@ action

\((\begin{array}{ll}0, & 0\end{array})\)
\((-4.5,6.5)\)
\(r=3\ \text{sin}(2\theta )\)

@ action

Symmetry with respect to the x-axis, y-axis, and origin.
\({r}^{2}=9\ \text{cos}\ \theta\)
\(r=\text{cos}(\frac{\theta }{5})\)

@ action

Symmetric with respect to x-axis only.
\(r=2\ \text{sec}\ \theta\)
\(r=1+\text{cos}\ \theta\)

@ action

Symmetry with respect to x-axis only.
\(\theta =\frac{\pi }{4}\)

@ action

Line \(y=x\)
\(r=\text{sec}\ \theta\)
\(r=\text{csc}\ \theta\)

@ action

\(y=1\)

Symbols used here

\sqrt{x},\ \sqrt[n]{x}

square root, n-th root
The non-negative number whose square (n-th power) is x.

\pi

pi
Ratio of a circle's circumference to its diameter, 3.14159…

\theta

theta
The usual name for an angle.

imaginary unit
i² = −1.

\frac{\partial f}{\partial x}

partial derivative
Derivative with respect to x, holding the other variables fixed.

\nabla f

gradient (nabla, del)
Vector of partial derivatives; points uphill.

\int f(x)\,dx,\ \int_a^b

integral
Antiderivative (indefinite) or signed area from a to b (definite).

\iint,\ \oint

double / contour integral
Integral over a region of the plane; integral around a closed curve.

\mathbf{v},\ \vec{v}

vector
A quantity with magnitude and direction; a column of numbers.

\mathbf{u} \cdot \mathbf{v},\ \|\mathbf{v}\|

dot product, norm
Σ u_i v_i; the length of v, √(v·v).

How to: Polar Coordinates

Locate points in a plane by using polar coordinates.
Convert points between rectangular and polar coordinates.
Sketch polar curves from given equations.
Convert equations between rectangular and polar coordinates.
Identify symmetry in polar curves and equations.
Use
Use
Use

Questions people ask

What is a partial derivative?

The ordinary derivative with respect to one variable while every other variable is frozen — the slope of the surface in one coordinate direction.

What does the gradient point at?

Uphill: the direction of steepest increase, with length equal to that steepest slope. It is perpendicular to the level curves.

QDialogButtonBox

Parts of this page are adapted from OpenStax Calculus Volume 2 (CC BY-NC-SA 4.0), OpenStax Calculus Volume 3 (CC BY-NC-SA 4.0). Condensed and re-explained here; errors are ours.

@ action Multivariable Calculus

Functions of several variables Partial derivatives and the gradient Optimisation in several variables Double and triple integrals Vector fields, line integrals and the big theorems

\(\theta\)	\(r\)
\(0\)	\(0\)
\(\frac{\pi }{6}\)	\(\frac{3\sqrt{3}}{2}\approx 2.6\)
\(\frac{\pi }{4}\)	\(3\)
\(\frac{\pi }{3}\)	\(\frac{3\sqrt{3}}{2}\approx 2.6\)
\(\frac{\pi }{2}\)	\(0\)