# Lagrange multiplier method

1. Mar 9, 2007

### sara_87

Question:

Use Lagrange multiplier method to determine the point on the curve
y=1-$$x^2$$
that maximises the function f(x,y)=2x + y.
Hence find the maximum value of f.

Attempt at Solution:

Okay I used the Lagrange method to get a point on the curve and I got (1,0)

How do I find the maximum value of f though?

2. Mar 9, 2007

### cristo

Staff Emeritus
If you've found the values of x and y where the function has a maximum, then do you not just plug (1,0) into f(x,y) to obtain the value of the function at that point?

3. Mar 9, 2007

### Dick

Uh, substitute (1,0) into f?

4. Mar 9, 2007

i get (0,0)?

5. Mar 9, 2007

### Dick

f is a number. Not a point. And it's not 0.

6. Mar 9, 2007

### sara_87

i'm still stuck! i don't what to do lol
give me more tips before i give up on maths altogether!!
(does anyone else go through a phase when they just want to give up? lol)

7. Mar 9, 2007

### Dick

You say f(x,y)=2*x+y. You've found a solution (1,0) so x=1, y=0. What is f(x,y)? Don't get so flustered!

8. Mar 9, 2007

### sara_87

'Don't get so flustered!'

you don't know what kind of day i've had! lol

thank you for your help and time i'm sure it's not a hard question i'll think about it tomorrow when i feel more awake.

9. Mar 9, 2007

### Dick

Your annoyance is blocking you from seeing the obvious. That makes it a really good time to take a rest.