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$(a)$ Sketch the region of integration in the integral

$$\int_{y=-2}^{2} \int_{x=0}^{\sqrt{4-y^2}} x e^{{(4-x^{2})}^{3/2}} dx dy$$

By changing the order of integration, or otherwise, evaluate the integral.

$(b)$ Let $R$ be the region in the $x-y$ plane defined by $0 \leq x \leq y \leq 2x$, $1 \leq x+2y \leq 4$. Evaluate: $$\mathop{\int\int}_{R} \frac{1}{x} dx dy$$

I understand how to draw these but I am not sure how to caluculate the limts in either case (especially part $b$).

Can someone explain how we calculate the limits for integration? Once I know that I am sure I can integrate the function myself. Thanks!!

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You are asking too many homework questions. It won't particularly benefit you to get any particular question answered because you do not seem to have a strong grasp of the underlying material, so I would suggest that you go to a professor or TA and/or reread your textbook more thoroughly and then think harder about these questions on your own. Constantly tossing out homework questions is not the purpose of this site. –  Qiaochu Yuan May 21 '11 at 20:01
This is not a homeowrk question. Im preparing for an exam and understand the steps but Ive never had a quadrilateral area to work out. The only examples we were given were triangles. Therefore I still dont know how to find the limits. –  user4645 May 23 '11 at 10:24

1 Answer 1

up vote 1 down vote accepted

The first thing, as stated, is to sketch the region. Only then calculate the limits of integration. So, what does your sketch look like?

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+1: but it might be hard to show his/her sketch. –  Fabian May 21 '11 at 18:46
For part (a). Its a circle going through (0,2), (2,0), (-2,0) and (0,-2). So we are integrating that top half of the circle. But what do we take for x and y? Why do we switch the order? And part (B)'s limits make even less sense to me! :( –  user4645 May 21 '11 at 18:47
For part (a), this is sketch of the region. –  Fabian May 21 '11 at 18:50
Yep its not the sketch thats the problem. Its the limits. What do we fix? x or y? what values? How do we calulate the x limits? Thats what confusing me. Can you explain it in general what o do so that I know for other questions? Thanks. –  user4645 May 21 '11 at 18:51
@user4645: First identify and sketch carefully your region. Then suppose you are integrating first with respect to $y$. Then $y$ goes from "bottom curve" to top curve." So if bottom curve is $y=f(x)$ and top curve is $y=g(x)$, integrate with respect to $y$ from $f(x)$ to $g(x)$. Then integrate with respect to $x$ from the first $x$ to the last. Sometimes, as in (b), top and bottom curves change. Then break up your region into parts where they don't change. In (b), $3$ parts will do the job. You really need someone at a blackboard pointing, or a video. This stuff is very visual. –  André Nicolas May 21 '11 at 20:57

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