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Mathematical Expression Editor
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Objectives:
1.
Be comfortable working in polar coordinates.
2.
Know how to transform a double integral in Cartesian coordinates
into a double integral in polar coordinates.
3.
Understand what the scaling factor is when moving to polar
coordinates.
Recap Video
The following video recaps the ideas of the section.
To recap:
Suppose is a function, and is a region in the -plane corresponding
to a region in polar coordinates. Then
Example Video
Below is a video working through an example of double integrals in polar
coordinates.
If is the region , , evaluate
Compute the integral of over the region which is the portion of in the first
quadrant using polar coordinates.
Steps:
Draw the region. The region looks like:
Write the integrand in polar coordinates:
The region in polar coordinates can be described as:
The double integral in polar is therefore:
Evaluate the integral to get:
Compute the integral of over the region bounded by , , and .
Again, draw
the region, which is shown in the figure.
After drawing the region:
Transform the integrand. In polar,
Figure out the bounds for and . Again, drawing rays out from the
origin, we see goes from the origin to the outer circle, which has
polar equation . Therefore
Also note that goes from the line to the line . The line has
polar equation (give answer between and , and the line has polar
equation .
The setup of the double integral in polar coordinates should be:
Evaluating this integral, we get:
Compute the integral of over the region inside the circle of radius
centered at .
The setup of the double integral in polar coordinates
is:
This integral evaluates to:
Compute the area of the region outside and inside .
The graph of the region
is shown below.
To get the area, we want to integrate , which in polar is
To get the bounds for and , first draw rays from the origin outward. Notice
that they enter the region at the circle , which has polar equation , and the
leave the region at the circle , which has polar equation . This gives us the
bounds. To get , notice that will sweep between the two intersection points
of the two circles. They intersect when intersects , so when . This
gives the values and . Therefore, the setup for the double integral
is
This evaluates to .
Set up the following double integral in polar coordinates: . The integral becomes:
Find the volume of the solid inside bounded above by and below by the
-plane.
If we are going to evaluate this as a double integral , we want to let .
The region in the -plane we want is the portion of where , which can be
described in polar as:
The double integral therefore becomes:
which evaluates to:
Start typing the name of a mathematical function to automatically insert it.
(For example, "sqrt" for root, "mat" for matrix, or "defi" for definite integral.)
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Start typing the name of a mathematical function to automatically insert it.
(For example, "sqrt" for root, "mat" for matrix, or "defi" for definite integral.)