1. Lagrange Multiplier (difficult?)

f(x,y) = d² = x² + y²
g(x,y) = 3x²-4xy+6y² = 140

Then by my calculations,

2x = λ(6x-4y) (eq.1)
2y = λ(-4x+12y) (eq.2)

rearranging 1 gives:
λ = 2x/(6x-4y) (eq.3)
rearranging 2 gives:
λ = 2y/(-2x+6y) (eq.4)

then by cross multiplying,
-2x² + 6xy = 3xy - 2y²

taking all terms to left hand side,
-2x² +3xy + 2y² = 0
(-2x - y)(x - 2y)=0
therefore x = y/-2 or x = 2y

This is where I am stuck, now I have a relationship between x and y, what do I do next??

2. Originally Posted by sk1001
f(x,y) = d² = x² + y²
g(x,y) = 3x²-4xy+6y² = 140

Then by my calculations,

2x = λ(6x-4y) (eq.1)
2y = λ(-4x+12y) (eq.2)

rearranging 1 gives:
λ = 2x/(6x-4y) (eq.3)
rearranging 2 gives:
λ = 2y/(-2x+6y) (eq.4)

then by cross multiplying,
-2x² + 6xy = 3xy - 2y²

taking all terms to left hand side,
-2x² +3xy + 2y² = 0
(-2x - y)(x - 2y)=0
therefore x = y/-2 or x = 2y

This is where I am stuck, now I have a relationship between x and y, what do I do next??
I'm assuming your algebra and arithemetic is correct.

Substitute x = y/-2 or x = 2y into 3x²-4xy+6y² = 140 to get an equation with a single unknown ......

3. I'm fairly sure of my working to that point
Going on from your advice, say I taken the case x=y/-2 and sub into the constraint equation g(x) = 3x^2 - 4xy....

I return y=±4. Now I have a value of x and y to I find the corresponding lambda? I am not that familiar with lagrange multipliers and my textbook doesnt cover the topic greatly so if you could guide me through the method step by step it would be greatly appreciated!

4. Originally Posted by sk1001
I'm fairly sure of my working to that point
Going on from your advice, say I taken the case x=y/-2 and sub into the constraint equation g(x) = 3x^2 - 4xy....

I return y=±4. Now I have a value of x and y to I find the corresponding lambda? I am not that familiar with lagrange multipliers and my textbook doesnt cover the topic greatly so if you could guide me through the method step by step it would be greatly appreciated!
Let me guess ...... This is a question on a university multivariable calculus assignment, it's asking you to find the maximum and minimum distance of the surface 3x²-4xy+6y² = 140 from the origin, and it's due in the next 24 hours or so.

Why do you need to calculate the value of lambda? Get the values of x and y and hence the points (x, y) that give the extremum. Then test the nature of these solutions.