
Page 1 of 1

[ 13 posts ] 

Maths question I need answered pretty much now
Author 
Message 
Yazstromo
Joined: Sun Aug 12, 2007 12:17 am Posts: 634 Location: Australian Canada, eh? Gender:

Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 4:46 pm
Alright, I've been given a table of values, time, and the percentage corresponding to the time. (The percentage being y(t), our function). I had to rearrange y = 100/(1 + e^(c1 +c2t)), to isolate c1 + c2t, which I believe I did, rearranging it to c1 + c2t = ln(100/y + 1) Now, I have to find a 10x2 matrix, A, and column vector (read: 10x1 matrix), b, such that A multiplied by (c1 c2)' = b, where the elements of b are functions of y. (NOTE: (c1 c2)', for those who don't know, means turn the thing vertically. It's just easy horizontal notation of vectors. So, it's a 2x1 matrix, with c1 being above and c2 below.) Trouble is, I haven't a clue how to do that. Help?




Darth Citrus
Joined: Tue Aug 16, 2005 1:41 pm Posts: 2812 Location: Canada Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 4:50 pm
Well first, isn't it suppose to be c1 + c2t = ln(100/y  1) ?
Other than that, I don't think I can help that much. Almost failed this course when I took it...
_________________




Ghostra
Joined: Tue Mar 20, 2007 1:56 pm Posts: 2482 Location: Your Reflection. *snicker* Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 5:03 pm
Smack. I can't remember, sorry Yaz. _ I do recognize something here, though. hmmm.
_________________“Asphixation, defenstration, breathing wool, decapitation. Drinking from a lava pool, driving drunk to look so cool. Crazy bees sting you a lot, eating any rats you caught  all these ways that you can die. Don't forget poisonous pie!" Classic SmashQueen, Fri Aug 16, 2013 11:03 pm There's always another secret... ~ Mistborn, Sanderson




Yazstromo
Joined: Sun Aug 12, 2007 12:17 am Posts: 634 Location: Australian Canada, eh? Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 5:50 pm
Table:
t y 0 1 1 5 2 10 3 50 4 61 5 75 6 85 7 95 8 99 9 99.5




Ghostra
Joined: Tue Mar 20, 2007 1:56 pm Posts: 2482 Location: Your Reflection. *snicker* Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 6:06 pm
So, c is column, correct? I have to ask, because you're like in Australia, and we have a tendency to use different terms. _
_________________“Asphixation, defenstration, breathing wool, decapitation. Drinking from a lava pool, driving drunk to look so cool. Crazy bees sting you a lot, eating any rats you caught  all these ways that you can die. Don't forget poisonous pie!" Classic SmashQueen, Fri Aug 16, 2013 11:03 pm There's always another secret... ~ Mistborn, Sanderson




Yazstromo
Joined: Sun Aug 12, 2007 12:17 am Posts: 634 Location: Australian Canada, eh? Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 6:09 pm
...c? Oh, you mean c1 and c2! No, those are just letters. Like x and y. The point of this assignment is to figure out what those letters equal. To do that, I have to come up with a 10x2 matrix, etc.




Ghostra
Joined: Tue Mar 20, 2007 1:56 pm Posts: 2482 Location: Your Reflection. *snicker* Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 6:12 pm
Hmmm. I thought that you could put the columns as there own matrices (X) is one matrix, (Y) is another, and put those matrices into the equation...Time to google it.
_________________“Asphixation, defenstration, breathing wool, decapitation. Drinking from a lava pool, driving drunk to look so cool. Crazy bees sting you a lot, eating any rats you caught  all these ways that you can die. Don't forget poisonous pie!" Classic SmashQueen, Fri Aug 16, 2013 11:03 pm There's always another secret... ~ Mistborn, Sanderson




Darth Citrus
Joined: Tue Aug 16, 2005 1:41 pm Posts: 2812 Location: Canada Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 6:17 pm
Wait, so are you having problems setting up the matrix or how to solve for b?
_________________




Yazstromo
Joined: Sun Aug 12, 2007 12:17 am Posts: 634 Location: Australian Canada, eh? Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 6:20 pm
...Both? I just need the matrix. We're solving for (c1 c2)'.




Darth Citrus
Joined: Tue Aug 16, 2005 1:41 pm Posts: 2812 Location: Canada Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 6:25 pm
Well assuming you plug in the numbers, wouldn't your matrix be:
[A]
1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9
With [b]
4.615120517 3.044522438 2.397895273 1.098612289 0.970530501 0.84729786 0.777704569 0.719122667 0.698184975 0.695656592
_________________




Ghostra
Joined: Tue Mar 20, 2007 1:56 pm Posts: 2482 Location: Your Reflection. *snicker* Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 6:28 pm
That's what I was thinking, but how do you get the other matrix? Multiply the new one by the old one? (A.k.a. Multiply Mat. A x Mat. B=Mat C.) I thought you needed a new matrix too (as in, a new 10x2, matrix)
_________________“Asphixation, defenstration, breathing wool, decapitation. Drinking from a lava pool, driving drunk to look so cool. Crazy bees sting you a lot, eating any rats you caught  all these ways that you can die. Don't forget poisonous pie!" Classic SmashQueen, Fri Aug 16, 2013 11:03 pm There's always another secret... ~ Mistborn, Sanderson




Darth Citrus
Joined: Tue Aug 16, 2005 1:41 pm Posts: 2812 Location: Canada Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 6:31 pm
Oh, he needs to use the matlab function to do that. I can't remember what it is, something like [A]*[b] or the inverse of something. But I think those are the two matrices anyway.
I hated this math so much btw.
_________________




Yazstromo
Joined: Sun Aug 12, 2007 12:17 am Posts: 634 Location: Australian Canada, eh? Gender:

Re: Maths question I need answered pretty much now • Posted: Tue Apr 21, 2009 10:55 pm
Yeah, that was A, and b was comprised of the answers to that "ln(100/y  1)" expression. What's more, I was getting hung up on a phrase which, if I had've followed my gut instinct, would've gotten it.
See, what I was getting caught up on was "functions of y". Not y(t), but f(y)...i.e. the equation they had me find (c1 + c2t). Also, every instinct told me to make the second column of A the times in the table (i.e. 0, 1, 2, etc.). Thus, if I had realised that b were the values of f(y), I would easily have realised the first column of A was 1s.
Now, the problem is, finding the reduced row echelon form of [Ab] would be pointless, as I had far too many values. What I needed was to turn A into a nice square matrix. My friend, somehow, realised that multiplying A by its transpose (A') would give me a nice 2x2 matrix (A'*A is 2x10 * 10x2). This gave me some matrix...c. Then I multiplied b by A', and it gave me a 2x1 matrix, d (A'b = d). Now, I had cx = d. Finding the reduced row echelon of [cd] then proceeded to give me my values for c1 and c2.
So simple, in hindsight. Don't you agree?






Page 1 of 1

[ 13 posts ] 

Who is online 
Users browsing this forum: No registered users and 1 guest 

You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot post attachments in this forum

