POLAR EQUATIONS
Example 1:
Change the rectangular equation to polar form. Write the polar equation as r in terms of .
Knowing that and , we can convert as follows:
Now, we must write r in terms of .
In this case, this is done by factoring. We will factor r out of both expressions to get
Using the Zero Product Principle we get
or .
The graph of is the pole.
Since the pole is already included in (Hint: Let = 0), we don't need .
Therefore, all we need to do is write in terms of .
That is, .
Thus, the polar form of is .
WARNING:
It is incorrect to divide both sides of the equation by r to arrive at the solution !!! From algebra you should remember that we cannot divide by a variable UNLESS we are certain that this variable will NEVER take on the value of 0. In our case, we find thatr actually can take on this value!
Example 2:
Change the rectangular equation to polar form. Write the polar equation as r in terms of .
Knowing that and , we can convert as follows:
Now, we must write r in terms of . In this case, this is done by factoring. We will factor r out of both expressions to get
Next, we will factor r out of both expressions as follows:
Using the Zero Product Principle we get
or
The graph of is the pole. Since the pole is already included in the graph of (Hint: Let = 0), we don't need .
Therefore, all we need to do is write in terms of .
That is, .
Thus, the polar form of is .
Example 3:
Change the rectangular equation to polar form. Write the polar equation as r in terms of .
Knowing that , we can convert as follows:
It may seem to you that we have accomplished our goal, that is, we converted the rectangular equation to polar form. However, it is standard procedure to write r in terms of .
Now, we must write r in terms of . In this case, this is done by factoring as follows:
and (Notice that we are dealing with a Difference of Squares).
Using the Zero Product Principle we get
or
Next, we will rewrite the polar equation as r in terms of .
That is, or
Either of the two equations is correct. However, it is accepted mathematical practice to let the positive value,, represent in polar form.
Remember that the variables in a polar equation are r and ! Only having an r-value means that it is held constant while the angle can take on any value.
Example 4:
Change the rectangular equation to polar form. Write the polar equation as r in terms of .
Knowing that , we can convert as follows:
Now, we must write the polar equation as r in terms of . All we have to do is divide both sides of the equation by sin.
or (Both solutions are acceptable!)
Thus, the polar form of is either or .
Example 5:
Change the polar equation to rectangular form.
In this case, we will multiply both sides of the equation by r to get
Knowing that and , we can convert as follows:
Thus, the rectangular form of is .
Example 6:
Change the polar equation to rectangular form.
Since we do not have a formula that converts r to rectangular form, we will raise both sides to the second power to get
Now we can use the formula to get .
Thus, the rectangular form of is .
Example 7:
Change the polar equation to rectangular form.
Since we do not have a formula that converts to rectangular form, we will place the "tan" in front of both sides of the equation as follows:
We can use the formula to get
and or (Both solutions are acceptable!)
Thus, the rectangular form of is or .
Example 8:
Change the polar equation to rectangular form.
Since we do not have a formula that converts secant to to rectangular form, we will use a Reciprocal Identity as follows:
Next, let's multiply both sides by , so that we get
Knowing that , we can convert as follows:
Thus, the rectangular form of is .
Example 9:
Convert the polar equation to a rectangular equation.
Since we do not have a formula that converts r to rectangular form, we'll multiply both sides of the equation by the denominator of the term on the right of the equal sign to get
r(3 3cos) = 1
then 3r 3rcos = 1
and 3r = 1 + 3cos
Next, we'll square both sides to get
and given that and , we can now write
Thus, the rectangular form of is 9y2 = 6x + 1.