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Shaun2357
06-25-2011, 03:48 PM
An insurance company insures a large number of drivers. Let X be the
random variable representing the company's losses under collision insurance,
and let Y represent the company's losses under liability insurance. X and Y
have joint density function
fXY (x; y) =
 (2x +2 - y)/4 0 < x < 1; 0 < y < 2
0 otherwise
What is the probability that the total loss is at least 1 ?

They want P(X + Y >= 1)?

Sinkingfun
06-25-2011, 05:15 PM
draw a graph. X + Y > 1 is the same as y > 1-x. Then you just do calculus for the shaded area above the triangle in the graph. When I did it, I got 17/24.

Bounds can be y going from 1-x to 2 and x going from 0 to 1.

Or you could use the bounds x going from 0 to 1 and y going from 0 to 1-x. Once you arrive at the answer, do 1 - the answer.

Shaun2357
06-25-2011, 05:17 PM
draw a graph. X + Y > 1 is the same as y > 1-x. Then you just do calculus for the shaded area above the triangle in the graph. When I did it, I got 17/24.

Bounds can be y going from 1-x to 2 and x going from 0 to 1.

Or you could use the bounds x going from 0 to 1 and y going from 0 to 1-x. Once you arrive at the answer, do 1 - the answer.

That's exactly what I did. Then I guess it's an adding mistake. Thanks though

Sinkingfun
06-25-2011, 09:10 PM
it's best to not rush the calculus unless you are a speed demon.

Shaun2357
06-26-2011, 03:58 PM
True. Found out it was an adding mistake.

Shaun2357
06-27-2011, 01:25 AM
Problem 30.3
Let X and Y be random variables with joint pdf given by
fxy(x,y) =

6(1−y) 0≤x≤y≤1
0 otherwise
(a) Find P(X ≤ 3/4,Y ≥ 1/2)

I am getting a little confused here with respect to finding out a. A little help is all I need. I don't expect someone to work out the integral, that I can do myself. But what should I integrate from?

nonlnear
06-27-2011, 01:40 AM
If the answer isn't jumping off the page at you, it's usually best to draw a diagram of the domain of integration. When doing this forget about the calculus and just focus on getting the domain right. Once you have the diagram complete, then evaluate the integral.

Could you clarify what the lower bound on x is? Is it 0, or 6(1-y), the product (which is strictly speaking what you wrote, but likely not what you meant to write), or both?

Gandalf
06-27-2011, 06:11 AM
If the answer isn't jumping off the page at you, it's usually best to draw a diagram of the domain of integration. When doing this forget about the calculus and just focus on getting the domain right. Once you have the diagram complete, then evaluate the integral.:iatp:

In particular, here you should consider whether these points should be included in your integration. If you classify all these correctly, you probably have the correct region: (.4,.7), (.7,.4), (.6,.7), (.7,.6), (.4,.8),(.8,.4) I have 3 of those inside the region.

Could you clarify what the lower bound on x is? Is it 0, or 6(1-y), the product (which is strictly speaking what you wrote, but likely not what you meant to write), or both?I interpret what he wrote as 0 is the lower limit. 6(1-y) is the density, for combinations of x and y where there is density.

nonlnear
06-27-2011, 07:13 AM
I interpret what he wrote as 0 is the lower limit. 6(1-y) is the density, for combinations of x and y where there is density.

Now that you mention it, that seems the best interpretation. I got a funny character after f(x,y) which made a line break, making everything look like the bounds expression. If 6(1-y) is the density function that simplifies things a bit. If the bounds are 0 < x < y < 1 then you can construct a diagram as follow. For each inequality (look at the list below and make sure you understand why each of them is contained in that one bounds statement) set up a separate inequality:
0 ≤ x
x ≤ y
0 ≤ y
x ≤ 1
y ≤ 1
All of these must be satisfied simultaneously, so after plotting all of them there is only one region in that mess of lines that represents your domain.

Shaun2357
06-27-2011, 11:02 PM
Now that you mention it, that seems the best interpretation. I got a funny character after f(x,y) which made a line break, making everything look like the bounds expression. If 6(1-y) is the density function that simplifies things a bit. If the bounds are 0 < x < y < 1 then you can construct a diagram as follow. For each inequality (look at the list below and make sure you understand why each of them is contained in that one bounds statement) set up a separate inequality:
0 ≤ x
x ≤ y
0 ≤ y
x ≤ 1
y ≤ 1
All of these must be satisfied simultaneously, so after plotting all of them there is only one region in that mess of lines that represents your domain.

Sorry about that. I copied and pasted from a pdf file and didn't realize that it doesn't copy correctly.

Shaun2357
06-28-2011, 03:16 PM
Much easier when you graph it...all though, i'm no artist

You're suppose to integrate fxy(x,y) from 0 <= x <= y dx then 0.5 <= y <= 0.75 dy and then add the other piece that you don't need to integrate which is 0.5(0.25)(0.75)(1.5) = 0.140625. :)

lloopy
06-28-2011, 04:28 PM
True. Found out it was an adding mistake.

This is why you do the hundreds of problems in preparation for the actual exam: to reduce the likelihood of a silly mistake when you know how to do the problem.

It's frustrating (and embarrassing) to blow addition when your calculus is fundamentally sound.

Shaun2357
06-29-2011, 12:50 AM
This is why you do the hundreds of problems in preparation for the actual exam: to reduce the likelihood of a silly mistake when you know how to do the problem.

It's frustrating (and embarrassing) to blow addition when your calculus is fundamentally sound.

Couldn't have said it better myself. I'm only taking that test when my average score on practice exams is a 9 or higher

nonlnear
06-29-2011, 08:00 AM
This is why you do the hundreds of problems in preparation for the actual exam: to reduce the likelihood of a silly mistake when you know how to do the problem.

It's frustrating (and embarrassing) to blow addition when your calculus is fundamentally sound.
Calculus is always the easy part. It's usually algebra and geometry that lead to mistakes.

Shaun2357
06-29-2011, 10:38 PM
Calculus is always the easy part. It's usually algebra and geometry that lead to mistakes.

Very true. That's why the most difficult classes for a Math Major will be the Abstract Algebra.

nonlnear
07-01-2011, 05:30 PM
Very true. That's why the most difficult classes for a Math Major will be the Abstract Algebra.
That depends on one's program. There are some challenging topology and number theory courses out there too... :dsmile:

Actuarialsuck
07-01-2011, 06:01 PM
I thought nonlnear meant that the reason why people struggle with Calculus is due to algebraic mistakes (not abstract algebra but basic algebra you learn in High School). And picking one course or area from the "pure math" category is ... useless. For any class you pick from one category, another one will be worse.

nonlnear
07-01-2011, 06:05 PM
I thought nonlnear meant that the reason why people struggle with Calculus is due to algebraic mistakes (not abstract algebra but basic algebra you learn in High School).That is what I meant. And picking one course or area from the "pure math" category is ... useless. For any class you pick from one category, another one will be worse.
That's also what I was trying to get at by throwing out some other areas that many find challenging. I'm just not quite as verbose as you, but we've got the same perspective apparently. :toast:

Shaun2357
07-02-2011, 09:18 PM
That depends on one's program. There are some challenging topology and number theory courses out there too... :dsmile:

That's true. Perhaps I should have said that, In my opinion [...]. I did find Topology fun