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#1 2012-11-11 09:09:22

John92
Guest

Stuck trying to prove/show something

Hey guys, i'm doing an econ assignment but i'm either going about the question in completely the wrong way - or im stuck on some math that i'm unable to do. Assuming it's the second (because i dont just want to post the question and get you to do the whole thing for me), here's what im stuck on.

A = 9xy

B = 9x + 9y - 18xy

C = 9 - 9x - 9y + 9xy

D = 12xy - 6x - 6y + 6


I want to prove that there are no values of x and y (where x and y are both values between 0 and 1) for which D is the greatest.

Is that possible to do numerically? Only thing i can think of is having some software plot a 3d graph and show that the D surface is always below at least one other - obviously thats no good for helping me do my homework though lol.

#2 2012-11-11 09:16:16

bobbym
Administrator

Offline

Re: Stuck trying to prove/show something

Hi;

D surface is always below at least one other

I am not following you here. Please post the exact question. Wording is very important in mathematics.


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

#3 2012-11-11 09:21:42

John92
Guest

Re: Stuck trying to prove/show something

Sorry, maybe ignore that bit, i'm not sure that even makes sense anyway - i was just speculating about how i'd demonstrate that at least one of the values, A, B, C would be greater than D at every value of x and y.

#4 2012-11-11 09:39:30

bobbym
Administrator

Offline

Re: Stuck trying to prove/show something

If x,y were in the interval of 0 and 1?


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

#5 2012-11-11 09:47:20

John92
Guest

Re: Stuck trying to prove/show something

That's right

#6 2012-11-11 09:53:56

bobbym
Administrator

Offline

Re: Stuck trying to prove/show something

Hi;

A is out, it is not always greater than D in the closed interval (0,1).

B is out, it is not always greater than D in the closed interval (0,1).

C is out, it is not always greater than D in the closed interval (0,1).


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

#7 2012-11-11 10:04:55

anonimnystefy
Real Member

Offline

Re: Stuck trying to prove/show something

I don't think the point is to prove that one of those is always greater than D, but rather at least one of those will be greater than D for some x and y.


The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment

#8 2012-11-11 10:49:13

bobbym
Administrator

Offline

Re: Stuck trying to prove/show something

Hi;

that at least one of the values, A, B, C would be greater than D at every value of x and y

Looks like he is saying A or B or C is always greater in that interval.


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

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