# Linear algebra

Watch
Announcements

Page 1 of 1

Go to first unread

Skip to page:

Hello, I'm currently doing part b) and getting a little stuck.

My instinct says that I need to write 5 equations, such as x(1) = 2 + s + t and then use the 5 equations to find s, t and then x(1) to x(5)?

However, I'm having real problems eliminating anything... so perhaps I'm not doing what the question is asking?

Or am I meant to be setting x(1) to be 18 (15+2+1) = 2 + s + t?

Also I don't know how I'm to know that there is a unique solution via the inequalities, but I know there is one via the RREF as rk(A) = rk(A|b).

Thanks!!

0

reply

Report

#2

(Original post by

Hello, I'm currently doing part b) and getting a little stuck.

My instinct says that I need to write 5 equations, such as x(1) = 2 + s + t and then use the 5 equations to find s, t and then x(1) to x(5)?

**Bameron**)Hello, I'm currently doing part b) and getting a little stuck.

My instinct says that I need to write 5 equations, such as x(1) = 2 + s + t and then use the 5 equations to find s, t and then x(1) to x(5)?

**inequalties**, so I'd say your instincts are wrong. (I also have no idea where 2 + s + t came from!).

Your first inequality should be: "Since , and ".

For how to proceed after you have the 5 inequalities, you just have to look for ways to find conditions on s and t that eventually allow you to find them.

0

reply

Ahh I don't know what I'm doing... I've followed your example and completed the other 4 inequalities, but I don't really know what you mean by finding conditions for s and t other than that they are both greater than or equal to 0 and I don't know what using that fact does for me.

Unfortunately your hint of x(2) + x(3) >= 0 hasn't helped me much... x(2) + x(3) = -10t... so -10t >= 0.

Ahh I don't know what I'm doing... I've followed your example and completed the other 4 inequalities, but I don't really know what you mean by finding conditions for s and t other than that they are both greater than or equal to 0 and I don't know what using that fact does for me.

Unfortunately your hint of x(2) + x(3) >= 0 hasn't helped me much... x(2) + x(3) = -10t... so -10t >= 0.

Unfortunately your hint of x(2) + x(3) >= 0 hasn't helped me much... x(2) + x(3) = -10t... so -10t >= 0.

(Original post by

Well, the question explictly tells you to form 5

Your first inequality should be: "Since , and ".

For how to proceed after you have the 5 inequalities, you just have to look for ways to find conditions on s and t that eventually allow you to find them.

**DFranklin**)Well, the question explictly tells you to form 5

**inequalties**, so I'd say your instincts are wrong. (I also have no idea where 2 + s + t came from!).Your first inequality should be: "Since , and ".

For how to proceed after you have the 5 inequalities, you just have to look for ways to find conditions on s and t that eventually allow you to find them.

Unfortunately your hint of x(2) + x(3) >= 0 hasn't helped me much... x(2) + x(3) = -10t... so -10t >= 0.

0

reply

Report

#4

(Original post by

Ahh I don't know what I'm doing... I've followed your example and completed the other 4 inequalities, but I don't really know what you mean by finding conditions for s and t other than that they are both greater than or equal to 0 and I don't know what using that fact does for me.

Unfortunately your hint of x(2) + x(3) >= 0 hasn't helped me much... x(2) + x(3) = -10t... so -10t >= 0.

**Bameron**)Ahh I don't know what I'm doing... I've followed your example and completed the other 4 inequalities, but I don't really know what you mean by finding conditions for s and t other than that they are both greater than or equal to 0 and I don't know what using that fact does for me.

Unfortunately your hint of x(2) + x(3) >= 0 hasn't helped me much... x(2) + x(3) = -10t... so -10t >= 0.

0

reply

(Original post by

If -10t >=0, what can we say about t? And what else do we know about t? So...

**DFranklin**)If -10t >=0, what can we say about t? And what else do we know about t? So...

0

reply

Report

#6

(Original post by

If -10t>= 0 then t has to be negative to be greater than 0, but we also know t is greater than 0, so we have a contradiction?

**Bameron**)If -10t>= 0 then t has to be negative to be greater than 0, but we also know t is greater than 0, so we have a contradiction?

0

reply

Ok so that leaves me with x(5) = 0, x(2) + x(3) = 0

from x(1), 2+s >= 0 so s >= -2, but from x(4), 4s >= 0, so s>=0, which I already knew from the question.

from x(1), 2+s >= 0 so s >= -2, but from x(4), 4s >= 0, so s>=0, which I already knew from the question.

Last edited by Bameron; 2 years ago

0

reply

Having s also = 0 give me the correct answer, but I don't know how to prove s = 0 from the other side, just that I know s>=0

0

reply

Report

#9

(Original post by

Having s also = 0 give me the correct answer, but I don't know how to prove s = 0 from the other side, just that I know s>=0

**Bameron**)Having s also = 0 give me the correct answer, but I don't know how to prove s = 0 from the other side, just that I know s>=0

You can show that must take on a different specific value though. Notice that the equations for are off by a multiple of -1 from each other (after imposing the condition ). For them both to be greater than (or equal to) 0, must take on a specific value which you can determine.

Last edited by RDKGames; 2 years ago

0

reply

Ok s=4/7 and this give me the correct portion sizes. Thank you both for being so patient with me, I'm going to run through the question again to make sure I understand everything.

0

reply

X

Page 1 of 1

Go to first unread

Skip to page:

### Quick Reply

Back

to top

to top