- Convert rectangular coordinates to polar coordinates.
- Convert equations given in rectangular form to equations in polar form and vice versa.
Polar to Rectangular
Just as and are usually used to designate the rectangular coordinates of a point, and are usually used to designate the polar coordinates of the point. is the distance of the point to the origin. is the angle that the line from the origin to the point makes with the positive axis. The diagram below shows both polar and Cartesian coordinates applied to a point . By applying trigonometry, we can obtain equations that will show the relationship between polar coordinates and the rectangular coordinates
The point has the polar coordinates and the rectangular coordinates .
These equations, also known as coordinate conversion equations, will enable you to convert from polar to rectangular form.
Example 1: Given the following polar coordinates, find the corresponding rectangular coordinates of the points:
a) For and
The rectangular coordinates of are approximately .
b) For and
The rectangular coordinates of are or approximately .
In addition to writing polar coordinates in rectangular form, the coordinate conversion equations can also be used to write polar equations in rectangular form.
Example 2: Write the polar equation in rectangular form.
The equation is now in rectangular form. The and have been replaced. However, the equation, as it appears, does not model any shape with which we are familiar. Therefore, we must continue with the conversion.
The rectangular form of the polar equation represents a circle with its centre at (2, 0) and a radius of 2 units.
This is the graph represented by the polar equation for or the rectangular form
Example 3: Write the polar equation in rectangular form.
Rectangular to Polar
When converting rectangular coordinates to polar coordinates, we must remember that there are many possible polar coordinates. We will agree that when converting from rectangular coordinates to polar coordinates, one set of polar coordinates will be sufficient for each set of rectangular coordinates. Most graphing calculators are programmed to complete the conversions and they too provide one set of coordinates for each conversion. The conversion of rectangular coordinates to polar coordinates is done using the Pythagorean Theorem and the Arctangent function. The Arctangent function only calculates angles in the first and fourth quadrants so radians must be added to the value of for all points with rectangular coordinates in the second and third quadrants.
In addition to these formulas, is also used in converting rectangular coordinates to polar form.
Example 4: Convert the following rectangular coordinates to polar form.
Solution: For and . The point is located in the fourth quadrant and .
The polar coordinates of are .
For and . The point is located in the third quadrant and .
The polar coordinates of are
To write a rectangular equation in polar form, the conversion equations of and are used.
Example 5: Write the rectangular equation in polar form.
Solution: Remember and .
Example 6: Write the rectangular equation in polar form.
Solution: Remember and .
If the graph of the polar equation is the same as the graph of the rectangular equation, then the conversion has been determined correctly.
The rectangular equation represents a circle with center (2, 0) and a radius of 2 units. The polar equation is a circle with center (2, 0) and a radius of 2 units.
Converting Using the Graphing Calculator
You have learned how to convert back and forth between polar coordinates and rectangular coordinates by using the various formulae presented in this lesson. The TI graphing calculator allows you to use the angle function to convert coordinates quickly from one form to the other. The calculator will provide you with only one pair of polar coordinates for each pair of rectangular coordinates.
Example 7: Express the rectangular coordinates of as polar coordinates.
Polar coordinates are expressed in the form . An angle can be measured in either degrees or radians, and the calculator will express the result in the form selected in the menu of the calculator.
Press and cursor down to Radian Degree. Highlight Degree. Press to return to home screen. To access the angle menu of the calculator press and this screen will appear:
Cursor down to 5 and press . The following screen will appear . Press -3, 7) and the value of will appear . Access the angle menu again by pressing . When the angle menu screen appears, cursor down to 6 and pres or press 6 on the calculator. The screen will appear. Press -3, 7) and the value of will appear.
This procedure can be repeated to determine the rectangular coordinates in radians. Before starting, press and cursor down to Radian Degree and highlight Radian.
Example 8: Express the polar coordinates of in rectangular form.
The angle is given in degrees so the mode menu of the calculator should also be set in degree. Therefore, press and cursor down to Radian Degree and highlight degree. Press to return to home screen. To access the angle menu of the calculator press and this screen will appear:
Cursor down to 7 and press or press 7 on the calculator. The following screen will screen will appear: Press 300, 70) and the value of will appear Access the angle menu again by pressing . When the angle menu screen appears, cursor down to 8 and pres or press 8 on the calculator. The screen will appear. Press 300,70) and the value of will appear .
Points to Consider
- When we convert coordinates from polar form to rectangular form, the process is very straightforward. However, when converting a coordinate from rectangular form to polar form there are some choices to make. For example the point 0,1 could translate to or to , and so on.
- Are there any advantages to using polar coordinates instead of rectangular coordinates? List any situations in which this is the case. What types of curves are easier to draw with polar coordinates?
- List situations in which rectangular coordinates are preferable.
- For the following polar coordinates that are shown on the graph, determine the rectangular coordinates for each point.
- Write the following polar equations in rectangular form.
- Write the following rectangular points in polar form.
- Write the rectangular equations in polar form.