# Q about finding area with double/volume with triple integral

• I
• AutumnWater
In summary, when calculating area with a double integral or volume with a triple integral, the integrand will be 1. This means that the iterated integral will give the volume bounded by the graph of the function above the rectangular region defined by the limits of integration. The boundaries of the interval or region do not affect the value of the integral, only the integrand does. However, if the integrand becomes negative, it will affect the value of the integral. Therefore, negative regions must be taken into account when finding the area or volume, and splitting the integral may be necessary in order to calculate the negative area or volume correctly.
AutumnWater
So when finding the Area from a double integral; or Volume from a triple integral: If the curve/surface has a negative region: (for areas, under the x axis), (for volumes, below z = 0 where z is negative)

What circumstances allow the negative regions to be taken into account as positive when finding the area/volume without having to split the integral up and add a negative piece to it?

Does polar form specifically allow for this since r can not be negative? Or is a double integral for area, triple integral for volume sufficient for this regardless of the coordinate system used since they are the same dimensionality as the scalar in question? Or they all have to be split up in order to calculate the negative area/volume correctly regardless of which coordinate system we're using?

AutumnWater said:
So when finding the Area from a double integral; or Volume from a triple integral: If the curve/surface has a negative region: (for areas, under the x axis), (for volumes, below z = 0 where z is negative)
If you're calculating area with a double integral, the integrand will be 1. Similarly, if you're calculating the volume of a solid in a triple integral, the integrand will also be 1. The iterated integral ##\int_{y = c}^d~\int_{x = a}^b f(x, y) dx~dy## gives the volume bounded by the graph of z = f(x, y) above the rectangular region defined by the limits of integration. For "volume" I'm assuming that the graph of f does not dip below the x-y plane.
AutumnWater said:
What circumstances allow the negative regions to be taken into account as positive when finding the area/volume without having to split the integral up and add a negative piece to it?
None that I can think of.
AutumnWater said:
Does polar form specifically allow for this since r can not be negative?
r can be negative. For example, the point ##(-2, \pi/4)## lies in the third quadrant.
AutumnWater said:
Or is a double integral for area, triple integral for volume sufficient for this regardless of the coordinate system used since they are the same dimensionality as the scalar in question? Or they all have to be split up in order to calculate the negative area/volume correctly regardless of which coordinate system we're using?
If you blithely integrate over an interval where the integrand function dips below the x-axis (with an integrand y = f(x)) or below the x-y plane (with an integrand of z = f(x, y)), the value of the integral will be less than the actual area/volume.

Mark44 said:
If you're calculating area with a double integral, the integrand will be 1. Similarly, if you're calculating the volume of a solid in a triple integral, the integrand will also be 1. The iterated integral ##\int_{y = c}^d~\int_{x = a}^b f(x, y) dx~dy## gives the volume bounded by the graph of z = f(x, y) above the rectangular region defined by the limits of integration. For "volume" I'm assuming that the graph of f does not dip below the x-y plane.
If you blithely integrate over an interval where the integrand function dips below the x-axis (with an integrand y = f(x)) or below the x-y plane (with an integrand of z = f(x, y)), the value of the integral will be less than the actual area/volume.
Thanks, I realize this is the case for double integral volumes and non volume triple integrals.
But I was specifically thinking when the integrands are 1 for double integral areas and triple integral volumes with boundaries that are in the negative regions. Does this situation allow the (1 integrand) double integral area or triple integral volume to be equal to the actual integral value itself? Or these also need to be split up?

Edit: I think I get it somehow now, it shouldn't matter that the x/y boundaries have negative y values for a double integral finding the area since z = 1 is a flat plane on the z = 1 level curve, and the area covered by this curve is always positive regardless of whether the y boundaries have become negative or not because the integral is actually trying to find the volume of the slice with height 0 and in turn finds the area instead, the sign of this slice's value is based on the z axis not on y-axis right?

Last edited:
AutumnWater said:
Thanks, I realize this is the case for double integral volumes and non volume triple integrals.
But I was specifically thinking when the integrands are 1 for double integral areas and triple integral volumes with boundaries that are in the negative regions. Does this situation allow the (1 integrand) double integral area or triple integral volume to be equal to the actual integral value itself? Or these also need to be split up?
It doesn't matter if the boundaries of the interval (for single integral) or region (for double or triple integrals) include negative parts. It only matters whether the integrand becomes negative.
AutumnWater said:
Edit: I think I get it somehow now, it shouldn't matter that the x/y boundaries have negative y values for a double integral finding the area since z = 1 is a flat plane on the z = 1 level curve, and the area covered by this curve is always positive regardless of whether the y boundaries have become negative or not because the integral is actually trying to find the volume of the slice with height 0 and in turn finds the area instead, the sign of this slice's value is based on the z axis not on y-axis right?
Close, but not quite. The iterated integral ##\int_{y = c}^d~\int_{x = a}^b 1~dy~dx## gives the area of the region defined by the limits of integration. It doesn't matter if some or all of the limits of integration are negative, as long as ##c \le d## and ##a \le b##. The integrand, z = 1, determines a solid whose thickness is 1 (not a height of 0 as you said), so that the volume of this solid is numerically equal to the area of the region of integration. Only the units (area units vs. volume units) are different.

AutumnWater
Thanks, that clears it all up for me now.

## 1. How is a double integral used to find the area under a curve?

A double integral is used to find the area under a curve by calculating the sum of infinitely small rectangles that make up the region under the curve. It involves integrating the function over two variables, usually x and y, within a given bounds.

## 2. Can a triple integral be used to find the volume of a 3D object?

Yes, a triple integral can be used to find the volume of a 3D object by integrating the function over three variables, usually x, y, and z, within a given bounds. It is similar to a double integral, but in three dimensions.

## 3. What is the difference between a double integral and a triple integral?

The main difference between a double integral and a triple integral is the number of variables being integrated. A double integral integrates a function over two variables, while a triple integral integrates over three variables. This allows for the calculation of areas and volumes in two and three-dimensional space, respectively.

## 4. How do you determine the bounds for a double or triple integral?

The bounds for a double or triple integral depend on the shape and dimensions of the region being integrated. They can be determined by setting up the integral in either rectangular, polar, or cylindrical coordinates and considering the limits for each variable.

## 5. Are there any applications of double and triple integrals in real life?

Yes, double and triple integrals have many applications in real life, particularly in fields such as physics, engineering, and economics. They can be used to calculate the volume of objects, the area under curves, and to solve optimization problems.

• Calculus
Replies
5
Views
2K
• Calculus
Replies
1
Views
2K
• Calculus
Replies
8
Views
2K
• Calculus
Replies
5
Views
1K
• Calculus
Replies
2
Views
2K
• Calculus
Replies
10
Views
3K
• Calculus
Replies
3
Views
582
• Calculus
Replies
5
Views
1K
• Calculus
Replies
3
Views
2K
• Calculus
Replies
3
Views
924