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Help me with Linear programming and Lagrangian method pretty please?

Okay, I think I understand the general gist of both methods but these three questions are seriously getting on my nerves. I have an exam in a couple of days and just stuck at the moment. Could anyone out there please go through these questions and go through the important steps?

Here are the three questions:

First Linear Programming question.



Second Linear Programming question



Finally the Lagrange multiplier question



Many advanced thanks to those who reply.
Reply 1
I don't like linear programming so..

I take it Calvin has £100 to spend on milk and cookies.

I'm interpreting your u as u=c0.6m0.4u=c^{0.6}m^{0.4}. Is that correct?

Using the price of milk and the price of cookies can you write the constraint function?
Reply 2
Original post by BabyMaths
I don't like linear programming so..

I take it Calvin has £100 to spend on milk and cookies.

I'm interpreting your u as u=c0.6m0.4u=c^{0.6}m^{0.4}. Is that correct?

Using the price of milk and the price of cookies can you write the constraint function?


The u is the utility yeah so its u=c0.6m0.4u=c^{0.6}m^{0.4}. The constraint should be 6c+2m=100. Income of £100 and price of cookie is £6 and milk is £2
Reply 3
Original post by Jim Carrey
The u is the utility yeah so its u=c0.6m0.4u=c^{0.6}m^{0.4}. The constraint should be 6c+2m=100. Income of £100 and price of cookie is £6 and milk is £2


OK, so we can write

g(c,m)=6c+2mu(c,m)=c0.6m0.4g(c,m)=6c+2m \\ u(c,m)=c^{0.6}m^{0.4}.

Now you need to solve

uc=λgcum=λgm3c+m=50u_c=\lambda g_c \\ u_m=\lambda g_m \\ 3c+m=50
Reply 4
Original post by BabyMaths
OK, so we can write

g(c,m)=6c+2mu(c,m)=c0.6m0.4g(c,m)=6c+2m \\ u(c,m)=c^{0.6}m^{0.4}.

Now you need to solve

uc=λgcum=λgm3c+m=50u_c=\lambda g_c \\ u_m=\lambda g_m \\ 3c+m=50

Still don't understand. Have a skip through this. http://www.youtube.com/watch?v=HLTyVlVQD8U

Is this woman doing it right? been following her technique.
Reply 5
Original post by Jim Carrey
Still don't understand. Have a skip through this. http://www.youtube.com/watch?v=HLTyVlVQD8U

Is this woman doing it right? been following her technique.


Yes. That video should sort you out. If it's not working out for you post your working.

If I remember correctly from yesterday the answers are:

maximum = 15 ish
m=10
c=20
Reply 6
Actually having taken a better look at that video I think the following is easier:
10x0.5y0.5+40λ=0\displaystyle 10 x^{-0.5} y ^{0.5} + 40 \lambda =0

10x0.5y0.5+120λ=0\displaystyle 10 x^{0.5} y ^{-0.5} + 120 \lambda =0

40x+120y=300000\displaystyle 40x + 120 y =300000

Write them in terms of x/y and divide through by 10.

(xy)0.5=4λ\displaystyle \left( \frac{x}{y}\right) ^{-0.5} =- 4 \lambda

(xy)0.5=12λ\displaystyle \left( \frac{x}{y}\right) ^{0.5} =- 12 \lambda

Divide the second equation by the first.

xy=3\frac{x}{y}=3 so x=3y\displaystyle x=3y

Use the constraint to get 24y=30000 so y=1250 and x=3750 and the maximum value for N is about 43300.
Reply 7
As a check on your answers to these questions..

If N=Axαy1α\displaystyle N=Ax^{\alpha}y^{1-\alpha} subject to ax+by=c\displaystyle ax+by=c then x=αca\displaystyle x=\frac{\alpha c}{a} and y=(1α)cb\displaystyle y=\frac{(1-\alpha)c}{b} will give the maximum value of N.

Of course you can't just use that in your exam if they want you to use Lagrange multipliers.
Reply 8
Original post by BabyMaths
Actually having taken a better look at that video I think the following is easier:
10x0.5y0.5+40λ=0\displaystyle 10 x^{-0.5} y ^{0.5} + 40 \lambda =0

10x0.5y0.5+120λ=0\displaystyle 10 x^{0.5} y ^{-0.5} + 120 \lambda =0

40x+120y=300000\displaystyle 40x + 120 y =300000

Write them in terms of x/y and divide through by 10.

(xy)0.5=4λ\displaystyle \left( \frac{x}{y}\right) ^{-0.5} =- 4 \lambda

(xy)0.5=12λ\displaystyle \left( \frac{x}{y}\right) ^{0.5} =- 12 \lambda

Divide the second equation by the first.

xy=3\frac{x}{y}=3 so x=3y\displaystyle x=3y

Use the constraint to get 24y=30000 so y=1250 and x=3750 and the maximum value for N is about 43300.

Man I'm such a thicko. Barely understand it. I'll just stick to the woman's method. I got C = 10 and m=20. Thanks for the help.
Reply 9
Original post by Jim Carrey
Man I'm such a thicko.


I doubt that. :smile:

Sadly, I also say this sort of thing to myself, too often.

Anyway, I'm glad you got there in the end. Good luck with the exam.

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