Question

Use Lagrange multipliers to find the maximum and minimum values of the function subject to the given constraint. f(x, y) = 2x2y, 2x2 + 4y2 = 12

Answer 1

Answer:

f(-2,-1) = -8 is the minimum value of f(x,y)

f(2,1) = 8 is the maximum value of f(x,y)

Step-by-step explanation:

f(x,y) = 2x²y under the constraint 2x² + 4y² = 12

Let g(x,y) = 2x² + 4y² - 12

df/dx = 4xy, df/dy = 2x², dg/dx = 4x and dg/dy = 8y

Now, using the principle of Lagrange multipliers,

df/dx + λdg/dx = 0 and df/dy + λdg/dy = 0.

Substituting the values of the variables, we have

df/dx + λdg/dx = 0    and df/dy + λdg/dy = 0.

4xy + 4xλ = 0      (1)           2x² + 8λy = 0  (2)

xy + xλ = 0

x(y + λ) = 0

x = 0 or y + λ = 0

Since x ≠ 0, y = -λ

Substituting y = -λ into (2), we have

2x² + 8λy = 0

2x² + 8λ(-λ) = 0

2x² - 8λ² = 0

2x² = 8λ²

x² = 4λ²

x = ±2λ  

Substituting the values of x and y into the constraint equation, we have

2x² + 4y² = 12

2(±2λ)² + 4(-λ)² = 12

2(4)λ² + 4λ² = 12

8λ² + 4λ² = 12

12λ² = 12

λ² = 1

λ = ±1

Substituting the value of  λ into x and y, we have

x = ±2λ = ±2(±1) = ±2

y = -λ = -(±1) = ±1

The minimum values of x and y are -2 and -1 respectively. Substituting these int f(x,y), we have

f(-2,-1) = 2(-2)²(-1) = 2 × 4 × (-1) = -8

So f(-2,-1) = -8 is the minimum value of f(x,y)

The maximum values of x and y are 2 and 1 respectively. Substituting these int f(x,y), we have

f(2,1) = 2(2)²(1) = 2 × 4 × 1 = 8

So f(2,1) = 8 is the maximum value of f(x,y)