# Double/triple intergral bounds

• Jacob87411
In summary: Cauchy-Riemann equations.In summary, to cover the region y^{2}=2x+6, you first graph the given curves, decide in what order to do the integrations, and identify the limits for each y. You then solve y2= 2x+6 and y= x-1 simultaneously, and use these solutions as limits for the outer and inner integrals, respectively.
Jacob87411
Hello,

I am having trouble understanding how to determine what is the lower and what is the upper bound in some calculus problems. For example:

Evaluate the double integral xydA, where D is the region bounded by the line y=x-1 and the parabola y^2=2x+6. Now you set it up to take the integral in terms of x first and the upper bound is y+1 and the lower is 1/2y^2+3...I guess I am just confused on why it isn't the other way around not just in this problem but in general i have problems recognizing which is which. thanks

You cannot express the relation $y^{2}=2x+6$ in such a way that y is a SINGLE function of the x!
Thus, you cannot really, use x as your independent variable (or rather, it becomes very hard to do so!).

However, you may use y as your independent variable.

I strongly advise you to actually DRAW the region of integration to get a feel for this, which obviously is something you didn't bother about doing.

For slightly "nastier", but still "nice" regions, we often manage to make a full change of variables, so that in the new set of variables, the domain is VERY nice (preferably a rectangle, since that's the simplest type of domain).

For still nastier regions, we get head-aches.

Jacob87411 said:
Hello,

I am having trouble understanding how to determine what is the lower and what is the upper bound in some calculus problems. For example:

Evaluate the double integral xydA, where D is the region bounded by the line y=x-1 and the parabola y^2=2x+6. Now you set it up to take the integral in terms of x first and the upper bound is y+1 and the lower is 1/2y^2+3...I guess I am just confused on why it isn't the other way around not just in this problem but in general i have problems recognizing which is which. thanks
Your objective is to "cover" the region. I recommend that you first graph the given curves. The next thing you have to do is decide in what order you want to do the integrations. You want to do the x- integration first (a good choice- I'll explain why in a moment). That means that the limits for the "outer", y, integration must be numbers while the limits for the "inner", x, integration may depend on y. In order to "cover" the region, the limits of integeration on y must go from the least possible value of y to the largest possible value of y. Obviously, from the graph, that occurs where the two curves cross. Solving y2= 2x+ 6 and y= x- 1 simultaneously, we find that the curves cross when y= -2 and y= 4. The limits on the outer integral are y= -2 and y= 1.
Now you have to decide on the limits for each y. Draw (or imagine) a horizontal line across your graph (representing a specific value of y). you see that the left endpoint of that line is on the parabola, the right endpoint on the line. Of course, the left x-value is the lower value, the right x-value is the higher value. On the parabola y2= 2x+ 6 so x= (1/2)y2- 3. That will be the lower limit of integration.
On the line y= x- 1 so x= y+ 1. That will be the upper limit of integration.
$$\int_{y= -2}^4\int_{x=\frac{1}{2}y^2- 3}^{y+1} xy dx dy$$
I recommend, by the way, actually writing the "x= " and "y= " in the limits as I did here as a reminder.

Suppose you had decided to integrate with respect to y first- in other words to do the integrals in the reverse order. Now we have to identify the lowest and highest values of x. We can see that the lowest value of x occurs at the vertex of the parabola, x= -3. The highest value is where the line and parabola cross at (5, 4). So the limits of integration on the outer integral will be x= -3, x= 5. Now imagine a vertical line across the graphs (representing a specific value of x). The upper end is on the parabola y2=- 2x+ 6 or $y= \sqrt{2x+6}$. But the parabola crosses the line at (-1, -2) so the lower end has two different expressions depending on whether x is less than or larger than -1. For x< -1, $y= -\sqrt{2x+6}$. For x> -1, y= x-1. That's why "x first
" was a good choice! However, we can break the region into two parts and say that the integral is
$$\int_{x=-3}^{-1}\int_{y= -\sqrt{2x+6}}^{\sqrt{2x+6}}xy dydx+ \int_{x=-1}^5\int_{y= x+1}^{\sqrt{2x+6}}xy dydx$$
You might try doing it both ways and see if they give the same answer.

In 2 dimensions, we have only those 2 orders: dxdy and dydx. In 3 dimension we have 3!= 6 possible orders: dxdydz, dxdzdy, dydxdz, dydzdx, dzdxdy, and dzdydx. Determining the limits of integration can be quite complicated!

so x= (1/2)y2- 3. That will be the lower limit of integration.
On the line y= x- 1 so x= y+ 1. That will be the upper limit of integration.

Thats the part I don't understand. How do you know which is which, which is upper and which is lower.

The difference function between the limit functions is continuous, hence, between two zeros, the difference function must necesssarily be single-signed. Pick a y in between, and find your sign.

Better yet, actually draw the graph!

## 1. What is the purpose of using double/triple integral bounds?

Double and triple integral bounds are used to determine the limits of integration for a particular function or region in multiple dimensions. They allow us to calculate the volume, area, or other quantities in a given space.

## 2. How do you determine the correct bounds for a double/triple integral?

The bounds for a double or triple integral are determined by the geometry of the region being integrated over. This can be visualized using graphs or sketches, and the bounds can be found by identifying the limits of integration for each variable.

## 3. Can the bounds for a double/triple integral change depending on the order of integration?

Yes, the bounds for a double/triple integral can change depending on the order of integration. This is because the limits of integration for each variable depend on the values of the other variables, and changing the order can affect the values of these limits.

## 4. What is the difference between the upper and lower bounds in a double/triple integral?

The upper bounds in a double/triple integral represent the highest value that a variable can take, while the lower bounds represent the lowest value. These bounds define the range of integration for each variable and are used to determine the volume or area of a given region.

## 5. Can you have negative bounds in a double/triple integral?

Yes, it is possible to have negative bounds in a double/triple integral. The bounds are determined by the limits of integration for each variable and can take on any value, including negative values. However, it is important to choose the correct orientation and order of integration to avoid any confusion or errors in the calculation.

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