Reversing Order of Integration for Double Integral Problem

In summary, it is important to remember that the anti-derivative of 1/(1+y^2) is not a logarithm, but rather arctan(y)+C. To solve this problem, it is best to reverse the order of integration and integrate arctan(1)-arctan(x^2) instead. This is a difficult problem, but with the help of Wolfram, it can be solved eventually.
  • #1
emelie_earl
3
0
8yv6i.png


My idea was that the limits are
BrQKm.png


and that the anti-derivative of dy was

xlog(1+y^2)

but that seems wrong...

maybe use these limits instead
YKvJd.png


and start with dx?

gives us
rDFwG.png


then we take dy

rgKWx.png


guess, i figured it out eventually with the help of wolfram with the last integration
 
Last edited:
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  • #2
The reason your 1st attempt didn't work is that [tex]\int\frac{1}{1+y^2}\,dy=\tan^{-1}(y)+C\,.[/tex]
 
  • #3
emelie_earl said:
8yv6i.png








My idea was that the limits are
BrQKm.png


and that the anti-derivative of dy was

xlog(1+y^2)

but that seems wrong...
Yes, it is wrong. It is standard "Calculus I" mistake to treat a function of the variable as if it were just the variable but you should be past that by the time you are doing multiple integrals. "[itex]1/(1+ y^2)[/itex]" is NOT the same as [itex]1/y[/itex] and its anti-derivative is not a logarithm. The anti-derivative of [itex]1/(1+ y^2)[/itex] is [itex]arctan(y)+ C[/itex]. That's a standard anti-derivative that you should have memorized.

maybe use these limits instead


YKvJd.png


and start with dx?

gives us
rDFwG.png


then we take dy

rgKWx.png


guess, i figured it out eventually with the help of wolfram with the last integration
Yes, reversing the order of integration is the best way to handle this one. Integerating [itex]arctan(1)- arctan(x^2)= \pi/4- arctan(x^2)[/itex] is likely to be very difficult!
 
  • #4
HallsofIvy said:
...
Yes, reversing the order of integration is the best way to handle this one. Integerating [itex]arctan(1)- arctan(x^2)= \pi/4- arctan(x^2)[/itex] is likely to be very difficult!
Yes. I agree !
 

Related to Reversing Order of Integration for Double Integral Problem

1. What is a double integral problem?

A double integral problem is a type of mathematical problem involving the integration of a function over a two-dimensional area or region. It is often used to calculate the volume under a surface or to find the area between two curves.

2. What is the difference between a single and a double integral?

A single integral involves the integration of a function along a one-dimensional interval, while a double integral involves the integration of a function over a two-dimensional region. In other words, a double integral is like the extension of a single integral into another dimension.

3. How do you solve a double integral problem?

The solution to a double integral problem involves breaking down the problem into smaller, simpler integrals using one of several methods, such as iterated integration, polar coordinates, or using a change of variables. The resulting integrals are then solved using basic integration techniques.

4. What are the applications of double integrals?

Double integrals have many applications in mathematics, physics, and engineering. They are used to calculate volumes, areas, and averages, as well as to solve problems involving mass, force, and probability in two-dimensional systems. They are also used in multivariable calculus and vector calculus.

5. Are there any common mistakes when solving double integral problems?

Yes, some common mistakes when solving double integral problems include incorrect setup of the integral, incorrect choice of limits of integration, and errors in the calculation of the integrals. It is important to carefully follow the steps and check for errors, especially when changing variables or using non-standard methods of solving the integral.

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