Math Problem Statement

Find the extreme value of the function f(x, y) = 2x^2 + 3y^2 - 4x - 5, on the region x^2 + y^2 ≤ 16.

Solution

The problem is asking us to find the extreme values of the function:

f(x,y)=2x2+3y24x5f(x, y) = 2x^2 + 3y^2 - 4x - 5

subject to the constraint:

x2+y216.x^2 + y^2 \leq 16.

Steps to Solve:

  1. Find the critical points of f(x,y)f(x, y):

    • Compute the partial derivatives of f(x,y)f(x, y): fx=x(2x2+3y24x5)=4x4,f_x = \frac{\partial}{\partial x}(2x^2 + 3y^2 - 4x - 5) = 4x - 4, fy=y(2x2+3y24x5)=6y.f_y = \frac{\partial}{\partial y}(2x^2 + 3y^2 - 4x - 5) = 6y.
    • Set fx=0f_x = 0 and fy=0f_y = 0 to find critical points: 4x4=0    x=1,4x - 4 = 0 \implies x = 1, 6y=0    y=0.6y = 0 \implies y = 0. Thus, the critical point is (1,0)(1, 0).
  2. Check the boundary x2+y2=16x^2 + y^2 = 16:

    • On the boundary, we use the constraint x2+y2=16x^2 + y^2 = 16 and substitute yy in terms of xx or vice versa.
    • Rewrite f(x,y)f(x, y) with the constraint and analyze its behavior using Lagrange Multipliers.
  3. Apply Lagrange Multipliers:

    • Define g(x,y)=x2+y216=0g(x, y) = x^2 + y^2 - 16 = 0.

    • Solve f=λg\nabla f = \lambda \nabla g, where: f=(4x4,6y),g=(2x,2y).\nabla f = (4x - 4, 6y), \quad \nabla g = (2x, 2y). This gives the system: 4x4=λ(2x),4x - 4 = \lambda (2x), 6y=λ(2y),6y = \lambda (2y), x2+y2=16.x^2 + y^2 = 16.

      • From 6y=λ(2y)6y = \lambda (2y), if y0y \neq 0, λ=3\lambda = 3.
      • From 4x4=λ(2x)4x - 4 = \lambda (2x), 4x4=6x4x - 4 = 6x, so x=2x = -2.

      Substituting x=2x = -2 into x2+y2=16x^2 + y^2 = 16: (2)2+y2=16    4+y2=16    y2=12    y=±23.(-2)^2 + y^2 = 16 \implies 4 + y^2 = 16 \implies y^2 = 12 \implies y = \pm 2\sqrt{3}.

      Additional points to evaluate are: (x,y)=(1,0),(2,23),(2,23).(x, y) = (1, 0), (-2, 2\sqrt{3}), (-2, -2\sqrt{3}).

  4. Evaluate f(x,y)f(x, y) at all critical points and boundary points:

    • At (1,0)(1, 0): f(1,0)=2(1)2+3(0)24(1)5=245=7.f(1, 0) = 2(1)^2 + 3(0)^2 - 4(1) - 5 = 2 - 4 - 5 = -7.
    • At (2,23)(-2, 2\sqrt{3}): f(2,23)=2(2)2+3(23)24(2)5,f(-2, 2\sqrt{3}) = 2(-2)^2 + 3(2\sqrt{3})^2 - 4(-2) - 5, =2(4)+3(12)+85=8+36+85=47.= 2(4) + 3(12) + 8 - 5 = 8 + 36 + 8 - 5 = 47.
    • At (2,23)(-2, -2\sqrt{3}): f(2,23)=47(same as above).f(-2, -2\sqrt{3}) = 47 \quad \text{(same as above)}.
  5. Determine the extreme values:

    • Minimum value: f(1,0)=7f(1, 0) = -7,
    • Maximum value: f(2,±23)=47f(-2, \pm 2\sqrt{3}) = 47.

Final Answer:

  • The maximum value is 4747.
  • The minimum value is 7-7.

Do you want a step-by-step explanation of any part? Here are five related questions to explore further:

  1. How do Lagrange Multipliers work, and why do we use them for constraints?
  2. Can we solve this problem graphically for better understanding?
  3. How do we verify if critical points are maxima, minima, or saddle points?
  4. What happens if the constraint is an inequality rather than equality?
  5. How does the method extend to three or more variables?

Tip: Always check both the interior critical points and the boundary when solving constrained optimization problems!

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Math Problem Analysis

Mathematical Concepts

Constrained Optimization
Partial Derivatives
Critical Points
Lagrange Multipliers

Formulas

Partial derivative of f(x, y): fx = ∂f/∂x, fy = ∂f/∂y
Lagrange Multiplier System: ∇f = λ∇g
Constraint equation: x^2 + y^2 = 16

Theorems

Lagrange Multiplier Method
Critical Point Theory

Suitable Grade Level

College Level (Calculus)