# Thread: method of Lagrange multipliers question

1. ## method of Lagrange multipliers question

Can someone tell me how to use method of Lagrange multipliers to do following question:

Use the method of Lagrange multipliers to find the critical points of the following constrained optimisation problems. Do not attempt to test the nature of these critical points.

(a) Optimise f(x, y) = x + y,
subject to the constraint: x^2 + y^2 - 4 = 0

(b) Optimise f(x, y, z) = x^2 + y^2 + z^2,
subject to the constraints: x + y + z = 1 and x + 2y + 3z = 2

2. Originally Posted by quah13579
Can someone tell me how to use method of Lagrange multipliers to do following question:

Use the method of Lagrange multipliers to find the critical points of the following constrained optimisation problems. Do not attempt to test the nature of these critical points.

(a) Optimise f(x, y) = x + y,
subject to the constraint: x^2 + y^2 - 4 = 0

(b) Optimise f(x, y, z) = x^2 + y^2 + z^2,
subject to the constraints: x + y + z = 1 and x + 2y + 3z = 2

(a)

$f = x+y + a ( x^2 + y^2 - 4 )$

we have

$f_x = 1 + 2ax = 0$ and

$f_y = 1 + 2ay = 0$

$x = y = - \frac{1}{2a}$

Sub. $x=y$ into the constraint $x^2 + y^2 = 4$

$2x^2 = 4 , x = \sqrt{2} ~ or~ -\sqrt{2}$

$(x,y) = ( \sqrt{2}, \sqrt{2}) ,( -\sqrt{2}, \sqrt{2}) ,( \sqrt{2}, -\sqrt{2}),( -\sqrt{2}, -\sqrt{2})$

(b)

$f= x^2 + y^2 + z^2 + a( x + y + z -1 ) + b( x + 2y + 3z -2)$

$f_x = 2x + a + b = 0$ (1)
$f_y = 2y + a + 2b = 0$ (2)
$f_x = 2z + a + 3b = 0$ (3)

$x = -\frac{1}{2} (a+b)$
$y = -\frac{1}{2} (a+2b)$
$z = -\frac{1}{2} (a+3b)$

It contructs a plane , also from the two constraints , we have three planes now , solve the system of three linear equations .

$(x,y,z) = ( \frac{1}{3} , \frac{1}{3} , \frac{1}{3} )$

3. Originally Posted by quah13579
Can someone tell me how to use method of Lagrange multipliers to do following question:

Use the method of Lagrange multipliers to find the critical points of the following constrained optimisation problems. Do not attempt to test the nature of these critical points.

(a) Optimise f(x, y) = x + y,
subject to the constraint: x^2 + y^2 - 4 = 0

(b) Optimise f(x, y, z) = x^2 + y^2 + z^2,
subject to the constraints: x + y + z = 1 and x + 2y + 3z = 2
A)

$1 = \lambda 2x$

$1 = \lambda 2y$

$x^2 + y^2 = 4$

$x = \frac{1}{2\lambda} = y$

so plug into your other equation

$x^2 + x^2 =4$

$2x^2 =4$

$x^2 = 2$

$x = ^+_- \sqrt{2}$

$(\sqrt{2},\sqrt{2}),(-\sqrt{2},\sqrt{2}),(-\sqrt{2},-\sqrt{2}),(\sqrt{2},-\sqrt{2})$