You will see during this lesson that points can be converted from rectangular form to polar form with a little algebra and trigonometry.
Can the equation of a shape be converted also? How about a circle, for instance?
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Khan Academy: Polar Coordinates 2
Guidance
Polar Form to Rectangular Form
Sometimes a problem will be given with coordinates in polar form but rectangular form may be needed.
To transform the polar point into rectangular coordinates: first identify (r, θ).
r = 4 and .
Second, draw a vertical line from the point to the polar axis (the horizontal axis). The distance from the pole to where the line you just drew intersects the polar axis is the x value, and the length of the line segment from the point to the polar axis is the y value.
These distances can be calculated using trigonometry:
x = r cos θ and y = r sin θ
and or
in polar coordinates is equivalent to in rectangular coordinates.
Rectangular Form to Polar Form
Going from rectangular coordinates to polar coordinates is also possible, but it takes a bit more work. Suppose we want to find the polar coordinates of the rectangular point (2, 2). To begin doing this operation, the distance that the point (2, 2) is from the origin (the radius, r) can be found by
The angle that the line segment between the point and the origin can be found by
Since this point is in the first quadrant (both the x and y coordinate are positive) the angle must be 45^{o} or radians. It is also possible that when tan θ = 1 the angle can be in the third quadrant, or radians. But this angle will not satisfy the conditions of the problem, since a third quadrant angle must have both x and y negative.
Note: when using to find the measure of θ you should consider, at first, the quotient and find the first quadrant angle that satisfies this condition. This angle will be called the reference angle, denoted θ_{ref}. Find the actual angle by analyzing which quadrant the angle must be given the signs of x and y.
Example A
Transform the polar coordinates to rectangular form
Solution:
and
and
and or
is equivalent to or in decimal form, approximately .
Example B
Find the polar coordinates for
and
Solution:
Draw a right triangle in standard form. Find the distance the point is from the origin and the angle the line segment that represents this distance makes with the +x axis:
And for the angle,
So, and we can look at the signs of x and y  (+, )  to see that since it is a 4^{th} quadrant angle.
The rectangular point is equivalent to the polar point .
Recall that when solving for θ, we used
or
We found
. BUT, θ could also be . You must examine the signs of each coordinate to see that the angle must be in the fourth quadrant in rectangular units or between and 2π in polar units. Of the two possible angles for θ, only is valid. Note that when you use tan^{1} on a calculator you will always get an answer in the range .
Example C
Convert the following rectangular coordinates to polar coordinates
a.
b.
Convert the following polar coordinates to rectangular coordinates:
c.
d.
Solutions:
a.
b.
c.
d.
Concept question wrapup: Equation of a circle x^{2} + y^{2} = k^{2} is the equation of a circle with a radius of k in rectangular coordinates. The equation of a circle is extremely simple in polar form. In fact, a circle on a polar graph is analogous to a horizontal line on a rectangular graph! You can transform this equation to polar form by substituting the polar values for x, y. Recall x = r cos θ and y = r sin θ. (r cos θ)^{2} + (r sin θ)^{2} = k^{2}, square the terms: r^{2} cos^{2} θ + r^{2} sin^{2} θ = k^{2}, factor the r^{2} from both terms on the left: r^{2} (cos^{2} θ + sin^{2} θ) = k^{2} recall the identity: cos^{2} θ + sin^{2} θ = 1 r^{2} = k^{2} Therefore: is an equation for a circle in polar units. When r is equal to a constant, the polar graph is a circle.


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Guided Practice
1) Change to rectangular coordinates.
2) Change to rectangular coordinates.
3) Express the equation in rectangular form:
4) Express the equation in rectangular form:
Answers
1) To change the description of the point to rectangular, first find the xvalue, then the yvalue, as follows:
xcoordinate:
 : the coordinate is found by multiplying by the cosine of
 : substitute the given information for and
ycoordinate:
 : the coordinate is found by multiplying by the sine of
 : substitute the given information for and
is the location of in rectangular form.
2) To change to rectangular coordinates, use the same process as Q #1:
xcoordinate:
 : substituting the values from the problem into
 :
ycoordinate
 : substituting the values from the problem into
 :
is the location of in rectangular form.
3) To express in rectangular form:
 : multiply both sides by
 : Using and
is the equation in rectangular form.
4) This one is easy:
 is the polar form of the equation for a circle
 : square both sides
 : Using and simplifying
is the equation in rectangular form.
Explore More
 How is the point with polar coordinates represented in rectangular coordinates?
Plot each point below in polar coordinates (r, θ). Then write the rectangular coordinates (x, y) for the point.
The rectangular coordinates (x, y) are given. For each question: a) Find two pairs of polar coordinates (r, θ), one with r > 0 and the other with r < 0. b) Express θ in radians, and round to the nearest hundredth.
Transform each polar equation to an equation using rectangular coordinates. Identify the graph, and give a rough sketch or description of the sketch.
Transform each rectangular equation to an equation using polar coordinates. Identify the graph, and give a rough sketch or description of the sketch.
Answers for Explore More Problems
To view the Explore More answers, open this PDF file and look for section 4.2.