This page covers double integrals in Cartesian coordinates. We start by discussing double integrals on rectangular regions. The next page discusses evaluating double integrals on nonrectangular regions.
Alternate Name For Double Integrals  Iterated Integrals 

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Double integrals are just integrals that are nested, i.e. there are two integrals, one inside of the other. The idea is to evaluate each integral separately, starting with the inside integral.
The discussion on this page is in two main parts based on the type of region described by the limits of integration.
If the limits of integration are constant, the region is rectangular. 

If the limits of integration are NOT constant, the region is nonrectangular. 
The basics of double integrals are discussed first in the context of rectangular regions. Before we get started on the details of evaluating double integrals, let's watch a video to help understand what double integrals represent.
video by Krista King Math 

Rectangular Regions
Rectangular regions in the xyplane are indicated by constants in the limits of integration, i.e. \( a \leq x \leq b \) and \( c \leq y \leq d\).
\[\displaystyle{ \int_{c}^{d}{ \int_{a}^{b}{g(x,y) ~ dx} ~dy } = \int_{c}^{d}{ \left[ \int_{a}^{b}{g(x,y) ~ dx}\right] ~dy}}\]
You can think about it this way.
First, evaluate the inside integral with respect to the inside variable, while holding the other variable constant. In the integral above, the inside variable is \(x\).
Second, take the result of the first integration and now integrate with respect to the outside variable. In the above integral, the outside variable is \(y\).
This integral, once evaluated, gives the area or volume above the xyplane with height \(g(x,y)\). We have more discussion of area and volume with double integrals on the area and volume application page. For now, we will just discuss how to evaluate a double integral.
Notice the order \(dx~dy\) or \(dy~dx\) is critical in determining which limits of integration go with which variable. In the example above, we integrated on the interval \([a,b]\) in the x direction and we evaluated on the interval \([c,d]\) in the y direction.
Okay, so that is a very basic introduction on how to look at a double integral. Let's watch a video to get started. This video explains the motivation behind double integrals, explains what double integrals are and includes the basics on how to set them up and evaluate them, using some basic examples.
video by Dr Chris Tisdell 

Are you ready for an example? Let's look at an easy double integral and take it stepbystep.
Evaluate \(\displaystyle{ \int_{1}^{2}{ \int_{2}^{4}{3xy^2 ~ dx} ~dy } }\)
Final Answer
42 

Problem Statement
Evaluate \(\displaystyle{ \int_{1}^{2}{ \int_{2}^{4}{3xy^2 ~ dx} ~dy } }\)
Solution
\(\displaystyle{ \int_{1}^{2}{ \int_{2}^{4}{3xy^2 ~ dx} ~dy } }\) 
\(\displaystyle{ \int_{1}^{2}{ \int_{2}^{4}{3xy^2 ~ dx} ~dy } }\) 
\(\displaystyle{ \int_{1}^{2}{ \left[ y^2 \int_{2}^{4}{3x ~ dx} \right] ~dy } }\) 
\(\displaystyle{ \int_{1}^{2}{ y^2 \left[ 3x^2/2 \right]_{x=2}^{x=4} ~dy } }\) 
\(\displaystyle{ \int_{1}^{2}{ y^2 \left[ 3(4)^2/2  3(2)^2/2 \right] ~dy } }\) 
\(\displaystyle{ \int_{1}^{2}{ y^2 \left[ 18 \right] ~dy } = \int_{1}^{2}{ 18y^2 ~dy } }\) 
\( \left[ 18y^3/3 \right]_{1}^{2} \) 
\( 18(2)^3/3  18(1)^3/3 \) 
\( 48  6 = 42 \) 
Notice in this solution, that after evaluating the inside integral, all the x's are gone, so that the outside integral is just a basic single variable definite integral with integrand \(18y^2\). This is due to the fact that we have constants as the limits of integration for the inside integral. Makes sense, right?
Final Answer
42 

Okay, let's watch a video tutorial. Here is another very good video explaining double integrals and how they work.
video by Dr Chris Tisdell 

In the above example, we were able to use a special technique that works only in specific instances. Here is a great video that explains exactly when and how to use it. Although we do not usually encourage learning special cases, this is easy to use, works all the time ( when used correctly ), saves time and it is easy to learn. So go for it!
video by Dr Chris Tisdell 

Notation
As discussed in the videos above, a double integral can be thought of as an integral over a region in the plane. This can be written several different ways. If you are given a region in the plane defined by the region R, all of the forms below describe the same integral.
the integral of \(f(x,y)\) over the region \( R = \{ (x,y) ~:~ a \leq x \leq b, ~ c \leq y \leq d \} \)  

\(\displaystyle{ \int_{c}^{d}{ \int_{a}^{b}{f(x,y) ~ dx} ~dy } }\) 
\(\displaystyle{ \iint\limits_R {f(x,y) ~ dA} }\) 
In the last form, \(dx~dy\) are combined to form \(dA\) and the integral signs are collapsed into the integral over R. This notation is very common and it is something you will see more of as you continue on in calculus.
Reversing Order of Integration
When the limits of integration of both integrals are constants (i.e. the region in the xyplane is rectangular), we can switch the limits of integration with no other changes to the integral, without affecting the result. Stop and think about this. This holds only when the limits of integration are constants.
So in the example above, we could have integrated with respect to \(y\) first, then with respect to \(x\) and arrived at the same answer. The equivalent integrals are
\(\displaystyle{ \int_{1}^{2}{ \int_{2}^{4}{3xy^2 ~ dx} ~dy } = \int_{2}^{4}{ \int_{1}^{2}{3xy^2 ~ dy} ~dx } }\)
Notice that when we switched the integral signs on the left, we also switched \(dx~dy \to dy~dx\). This is required since the intervals do not change, i.e. \([2,4]\) is in the \(x\) direction and \([1,2]\) is in the \(y\) direction. So we need to keep the same association in an equivalent integral. [Try working this second integral to convince yourself of this.]
Let's work some practice problems. After that, you will be ready for double integrals over nonrectangular regions.
Practice
Unless otherwise instructed, evaluate these integrals, giving your answers in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
\(\displaystyle{ \int_{1}^{3}{ \int_{0}^{1}{ (1+4xy) ~dx } ~dy } }\)
Problem Statement 

\(\displaystyle{ \int_{1}^{3}{ \int_{0}^{1}{ (1+4xy) ~dx } ~dy } }\)
Final Answer 

\( 10 \)
Problem Statement
\(\displaystyle{ \int_{1}^{3}{ \int_{0}^{1}{ (1+4xy) ~dx } ~dy } }\)
Solution
This is the same problem as 3512 except that the order of integration is reversed.
video by Michel vanBiezen 

Final Answer
\( 10 \)
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\(\displaystyle{ \int_{0}^{1}{ \int_{1}^{3}{ (1+4xy) ~dy } ~dx } }\)
Problem Statement 

\(\displaystyle{ \int_{0}^{1}{ \int_{1}^{3}{ (1+4xy) ~dy } ~dx } }\)
Final Answer 

\( 10 \)
Problem Statement
\(\displaystyle{ \int_{0}^{1}{ \int_{1}^{3}{ (1+4xy) ~dy } ~dx } }\)
Solution
Notice that this is the same problem as 3511 except that the order of integration is reversed. The answers are the same, as should be the case.
video by Michel vanBiezen 

Final Answer
\( 10 \)
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\(\displaystyle{ \int_{0}^{2}{ \int_{1}^{3}{ xy^2 ~dy } ~dx } }\)
Problem Statement 

\(\displaystyle{ \int_{0}^{2}{ \int_{1}^{3}{ xy^2 ~dy } ~dx } }\)
Final Answer 

\( 52/3 \)
Problem Statement
\(\displaystyle{ \int_{0}^{2}{ \int_{1}^{3}{ xy^2 ~dy } ~dx } }\)
Solution
In the second half of this video clip, he switches the limits of integration and shows that the answer will be the same.
video by The Organic Chemistry Tutor 

Final Answer
\( 52/3 \)
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\(\displaystyle{ \int_{0}^{2}{ \int_{0}^{\pi/2}{ x \sin y ~dy } ~dx } }\)
Problem Statement 

\(\displaystyle{ \int_{0}^{2}{ \int_{0}^{\pi/2}{ x \sin y ~dy } ~dx } }\)
Final Answer 

\( 2 \)
Problem Statement
\(\displaystyle{ \int_{0}^{2}{ \int_{0}^{\pi/2}{ x \sin y ~dy } ~dx } }\)
Solution
video by Michel vanBiezen 

Final Answer
\( 2 \)
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\(\displaystyle{ \int_{0}^{3}{ \int_{0}^{2}{ xy e^{x^2y} ~dy } ~dx } }\)
Problem Statement
\(\displaystyle{ \int_{0}^{3}{ \int_{0}^{2}{ xy e^{x^2y} ~dy } ~dx } }\)
Solution
video by The Organic Chemistry Tutor 

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\(\displaystyle{ \int_{1}^{1}{ \int_{2}^{2}{ x^2  y^2 ~dy ~ dx } } }\)
Problem Statement 

\(\displaystyle{ \int_{1}^{1}{ \int_{2}^{2}{ x^2  y^2 ~dy ~ dx } } }\)
Final Answer 

\( 8 \)
Problem Statement
\(\displaystyle{ \int_{1}^{1}{ \int_{2}^{2}{ x^2  y^2 ~dy ~ dx } } }\)
Solution
\(\displaystyle{ \int_{1}^{1}{ \int_{2}^{2}{ x^2  y^2 ~dy ~ dx } } }\) 
\(\displaystyle{ \int_{1}^{1}{ \left[ x^2 y  \frac{y^3}{3} \right]_{y=2}^{y=2} ~dx } }\) 
\(\displaystyle{ \int_{1}^{1}{ \left[ 2x^2  \frac{(2)^3}{3} \right]  \left[ 2x^2  \frac{(2)^3}{3} \right] ~dx } }\) 
\(\displaystyle{ \int_{1}^{1}{ 4x^2  \frac{16}{3} ~dx } }\) 
\(\displaystyle{ \left[ \frac{4x^3}{3}  \frac{16x}{3} \right]_{1}^{1} }\) 
\(\displaystyle{ \left[ \frac{4}{3}  \frac{16}{3} \right]  \left[ \frac{4(1)^3}{3}  \frac{16(1)}{3} \right] }\) 
\(\displaystyle{ \frac{8}{3}  \frac{32}{3} = \frac{24}{3} = 8 }\) 
Final Answer
\( 8 \)
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\(\displaystyle{ \int_{1}^{3}{ \int_{2}^{3}{ x^2  2xy + 2y^3 ~ dy } ~dx } }\)
Problem Statement
Evaluate the integral \(\displaystyle{ \int_{1}^{3}{ \int_{2}^{3}{ x^2  2xy + 2y^3 ~ dy } ~dx } }\), giving your answer in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
Solution
video by Dr Chris Tisdell 

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\(\displaystyle{ \int_{2}^{3}{ \int_{0}^{1}{ x^2 y ~dx } ~dy } }\)
Problem Statement
Evaluate the integral \(\displaystyle{ \int_{2}^{3}{ \int_{0}^{1}{ x^2 y ~dx } ~dy } }\), giving your answer in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
Solution
video by Dr Chris Tisdell 

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\(\displaystyle{ \int_{1}^{3}{ \int_{0}^{2}{ xy + x^2y^3 ~dy } ~dx } }\)
Problem Statement
Evaluate the integral \(\displaystyle{ \int_{1}^{3}{ \int_{0}^{2}{ xy + x^2y^3 ~dy } ~dx } }\), giving your answer in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
Solution
video by PatrickJMT 

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Evaluate \(\displaystyle{ \iint\limits_R { 2y  3x^2y^2 ~ dA } }\) over the region \( R = \{ (x,y) ~~ 0 \leq x \leq 1,~ 0 \leq y \leq 2 \} \)
Problem Statement 

Evaluate \(\displaystyle{ \iint\limits_R { 2y  3x^2y^2 ~ dA } }\) over the region \( R = \{ (x,y) ~~ 0 \leq x \leq 1,~ 0 \leq y \leq 2 \} \)
Final Answer 

\( 4/3 \)
Problem Statement
Evaluate \(\displaystyle{ \iint\limits_R { 2y  3x^2y^2 ~ dA } }\) over the region \( R = \{ (x,y) ~~ 0 \leq x \leq 1,~ 0 \leq y \leq 2 \} \)
Solution
video by The Organic Chemistry Tutor 

Final Answer
\( 4/3 \)
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Evaluate \(\displaystyle{ \iint\limits_R { (4x+y)^3 ~ dA } }\) over the region \( R = [0, 2] \times [0, 3] \)
Problem Statement 

Evaluate \(\displaystyle{ \iint\limits_R { (4x+y)^3 ~ dA } }\) over the region \( R = [0, 2] \times [0, 3] \)
Final Answer 

\( 3201/2 \)
Problem Statement
Evaluate \(\displaystyle{ \iint\limits_R { (4x+y)^3 ~ dA } }\) over the region \( R = [0, 2] \times [0, 3] \)
Solution
video by The Organic Chemistry Tutor 

Final Answer
\( 3201/2 \)
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\(\displaystyle{ \int_{1}^{4}{ \int_{\pi}^{3\pi/2}{ x \sin(xy) ~dx } ~dy } }\)
Problem Statement 

\(\displaystyle{ \int_{1}^{4}{ \int_{\pi}^{3\pi/2}{ x \sin(xy) ~dx } ~dy } }\)
Final Answer 

\( 1 \)
Problem Statement
\(\displaystyle{ \int_{1}^{4}{ \int_{\pi}^{3\pi/2}{ x \sin(xy) ~dx } ~dy } }\)
Solution
video by Maths With Jay 

Final Answer
\( 1 \)
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\(\displaystyle{ \int_{0}^{2}{ \int_{0}^{4}{ 3x + 4y ~dx } ~dy } }\)
Problem Statement
Evaluate the integral \(\displaystyle{ \int_{0}^{2}{ \int_{0}^{4}{ 3x + 4y ~dx } ~dy } }\), giving your answer in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
Solution
video by Krista King Math 

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\(\displaystyle{ \int_{1}^{2}{ \int_{1}^{3}{ 2x  7y ~dy } ~dx } }\)
Problem Statement
Evaluate the integral \(\displaystyle{ \int_{1}^{2}{ \int_{1}^{3}{ 2x  7y ~dy } ~dx } }\), giving your answer in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
Solution
video by Krista King Math 

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\(\displaystyle{ \int_{0}^{3}{ \int_{0}^{3}{ xy + 7x + y ~dx } ~dy } }\)
Problem Statement
Evaluate the integral \(\displaystyle{ \int_{0}^{3}{ \int_{0}^{3}{ xy + 7x + y ~dx } ~dy } }\), giving your answer in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
Solution
video by Krista King Math 

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Integrate \(\displaystyle{ f(x,y) = 2xy  3y^2 }\) over the region \( R=[1,1] \times [2,2] \)
Problem Statement
Integrate \(\displaystyle{ f(x,y) = 2xy  3y^2 }\) over the region \( R=[1,1] \times [2,2] \)
Solution
video by Krista King Math 

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\(\displaystyle{ \int_{1}^{4}{ \int_{1}^{2}{ \frac{x}{y} + \frac{y}{x} dy} ~dx } }\)
Problem Statement
Evaluate the integral \(\displaystyle{ \int_{1}^{4}{ \int_{1}^{2}{ \frac{x}{y} + \frac{y}{x} dy} ~dx } }\), giving your answer in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
Solution
video by Krista King Math 

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Evaluate the integral \(\displaystyle{ \iint\limits_R{ \frac{xy^2}{x^2+1} ~dA } }\) over the region \( R=\{(x,y)~:~ 0 \leq x \leq 1, ~ 3 \leq y \leq 3 \} \)
Problem Statement
Evaluate the integral \(\displaystyle{ \iint\limits_R{ \frac{xy^2}{x^2+1} ~dA } }\) over the region \( R=\{(x,y)~:~ 0 \leq x \leq 1, ~ 3 \leq y \leq 3 \} \)
Solution
video by Krista King Math 

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\(\displaystyle{ \int_{0}^{ \sqrt{2\pi/3}}{ \int_{0}^{\sqrt{7\pi/6}}{ xy ~ \sin(x^2+y^2) ~dx } ~dy } }\)
Problem Statement 

Evaluate the integral \(\displaystyle{ \int_{0}^{ \sqrt{2\pi/3}}{ \int_{0}^{\sqrt{7\pi/6}}{ xy ~ \sin(x^2+y^2) ~dx } ~dy } }\), giving your answer in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
Final Answer 

\( \sqrt{3}/8 \)
Problem Statement
Evaluate the integral \(\displaystyle{ \int_{0}^{ \sqrt{2\pi/3}}{ \int_{0}^{\sqrt{7\pi/6}}{ xy ~ \sin(x^2+y^2) ~dx } ~dy } }\), giving your answer in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.
Solution
video by MIP4U 

Final Answer
\( \sqrt{3}/8 \)
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\(\displaystyle{ \int_{1/2}^{1}{ \int_0^1{ \frac{y^2x^2}{(x^2+y^2)^2} ~dx } ~dy } }\)
Problem Statement 

\(\displaystyle{ \int_{1/2}^{1}{ \int_0^1{ \frac{y^2x^2}{(x^2+y^2)^2} ~dx } ~dy } }\)
Hint 

Use the trig substitution \( x = y\tan\theta \).
Problem Statement 

\(\displaystyle{ \int_{1/2}^{1}{ \int_0^1{ \frac{y^2x^2}{(x^2+y^2)^2} ~dx } ~dy } }\)
Final Answer 

\(\pi/4  \arctan(1/2)\)
Problem Statement
\(\displaystyle{ \int_{1/2}^{1}{ \int_0^1{ \frac{y^2x^2}{(x^2+y^2)^2} ~dx } ~dy } }\)
Hint
Use the trig substitution \( x = y\tan\theta \).
Solution
video by Krista King Math 

Final Answer
\(\pi/4  \arctan(1/2)\)
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Practice Instructions
Unless otherwise instructed, evaluate these integrals, giving your answers in exact terms. If it is not possible to evaluate the integral in the given form, try changing the order of integration.