Reversing order of integration

[Fusi]

Homework Statement

Reverse the order of integration:

\int_{0}^{1}\int_{y^2}^{2-y} f(x,y) \ dx\ dy This was on a test I took today. I was having trouble with it and was wondering how I should have gone about it?

 

[Mark44]

Homework Statement

Reverse the order of integration:

\int_{0}^{1}\int_{y^2}^{2-y} f(x,y) \ dx\ dy


This was on a test I took today. I was having trouble with it and was wondering how I should have gone about it?

Sketch a graph of the region over which integration is taking place. In the integral above, the inner integral runs from x = y² to x = 2 - y. The outer integral runs from y = 0 to y = 1. When you reverse the order of integration for this problem you will end up with two iterated integrals.

 

[Fusi]

So x=2-y and x=y^{2} so y=2-x y=\sqrt{x}

intersect at: 2-x=\sqrt{x} x=1 y=1

y at 0 is (0,2)

I got confused because at y=1,x=1 each side can be explained differently so my first attempt at a solutions was:

\int_{0}^{1}\int_{0}^{\sqrt{x}} f(x,y) \ dy\ dx + \int_{1}^{2}\int_{2-x}^{1} f(x,y) \ dy\ dx

I used some random logic in my head to get that and we've never seen anything like that in class before so I crossed it out and just wrote:

\int_{0}^{2}\int_{2-x}^{\sqrt{x}} f(x,y) \ dy\ dx

 

[Mark44]

So x=2-y and x=y^{2} so y=2-x y=\sqrt{x}

intersect at: 2-x=\sqrt{x} x=1 y=1

y at 0 is (0,2)

I got confused because at y=1,x=1 each side can be explained differently so my first attempt at a solutions was:

\int_{0}^{1}\int_{0}^{\sqrt{x}} f(x,y) \ dy\ dx + \int_{1}^{2}\int_{2-x}^{1} f(x,y) \ dy\ dx

This isn't correct, but it's closer than what you have below. Here's what you should get:
\int_{0}^{1}\int_{0}^{\sqrt{x}} f(x,y) \ dy\ dx + \int_{1}^{2}\int_{0}^{2 - x} f(x,y) \ dy\ dx

I used some random logic in my head to get that and we've never seen anything like that in class before so I crossed it out and just wrote:

\int_{0}^{2}\int_{2-x}^{\sqrt{x}} f(x,y) \ dy\ dx

 

[Fusi]

Yea I made a mistake on my last post. I actually did write my second bound as {0}\leq{y}\leq{2-x}

We've never seen a problem like that before so I got so nervous I just crossed the entire answer out. The answer I decided to write down in the last couple seconds was actually:

\int_{0}^{2}\int_{\sqrt{x}}^{2-x} f(x,y) \ dy\ dx

I kind of just tried to push the two integrals from my past answer together so I wouldn't have two iterated integrals.

 

[Mark44]

You understand why a single integral won't work, right?

 

[Fusi]

Yea, I have a bad habit of writing something that I know can't work when I get nervous.