Maximize 4x+ 2 y+ 2 z

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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Once I set up my lambda with either gradient I struggle with finding what point I should use in order to get my max value. If you could please show how you get your answer that would be very helpful. Thank you.
**Question 8**

Maximize \(4x + 2y + 2z\) on the sphere \(x^2 + y^2 + z^2 = 19\).

Options:
a) There is no maximum.
b) The maximum is \(\frac{19\sqrt{14}}{18}\).
c) The maximum is \(\frac{2\sqrt{14}}{3}\).
d) The maximum is \(-2\sqrt{114}\).
e) The maximum is \(2\sqrt{114}\).
f) None of these.

**Solution Steps:**

1. Defined function: \( f(x, y, z) = 4x + 2y + 2z \)
2. Gradient of \( f(x, y, z) = (4, 2, 2) \)

Steps involve using Lagrange multipliers:
- Define \( g(x, y, z) = x^2 + y^2 + z^2 - 19 \)
- Gradient of \( g(x, y, z) = (2x, 2y, 2z) \)
- Setting \(\nabla f = \lambda \nabla g\) leads to a system of equations:
  - \( 4 = 2\lambda x \)
  - \( 2 = 2\lambda y \)
  - \( 2 = 2\lambda z \)
  - Sphere constraint: \( x^2 + y^2 + z^2 = 19 \)

Solving the equations:
- From \( 2\lambda x = 4 \) and \( 2\lambda y = 2 \):
  - \( x = 2/\lambda \)
  - \( y = 1/\lambda \)
  - \( z = 1/\lambda \)
- Substitute into the sphere equation to solve for \( \lambda \).

This problem involves using calculus and linear algebra concepts to find the maximum value of a linear function constrained on a sphere, illustrating applications of optimization in multivariable calculus.
Transcribed Image Text:**Question 8** Maximize \(4x + 2y + 2z\) on the sphere \(x^2 + y^2 + z^2 = 19\). Options: a) There is no maximum. b) The maximum is \(\frac{19\sqrt{14}}{18}\). c) The maximum is \(\frac{2\sqrt{14}}{3}\). d) The maximum is \(-2\sqrt{114}\). e) The maximum is \(2\sqrt{114}\). f) None of these. **Solution Steps:** 1. Defined function: \( f(x, y, z) = 4x + 2y + 2z \) 2. Gradient of \( f(x, y, z) = (4, 2, 2) \) Steps involve using Lagrange multipliers: - Define \( g(x, y, z) = x^2 + y^2 + z^2 - 19 \) - Gradient of \( g(x, y, z) = (2x, 2y, 2z) \) - Setting \(\nabla f = \lambda \nabla g\) leads to a system of equations: - \( 4 = 2\lambda x \) - \( 2 = 2\lambda y \) - \( 2 = 2\lambda z \) - Sphere constraint: \( x^2 + y^2 + z^2 = 19 \) Solving the equations: - From \( 2\lambda x = 4 \) and \( 2\lambda y = 2 \): - \( x = 2/\lambda \) - \( y = 1/\lambda \) - \( z = 1/\lambda \) - Substitute into the sphere equation to solve for \( \lambda \). This problem involves using calculus and linear algebra concepts to find the maximum value of a linear function constrained on a sphere, illustrating applications of optimization in multivariable calculus.
Expert Solution
Step 1

We have to maximize f(x,y,z)=4x+2y+2z on the sphere x2+y2+z2=19.Take g(x,y,z)= x2+y2+z2-19=0Thus, given problem is equivalent to     Max  f(x,y,z)=4x+2y+2z    Subject to,      g(x,y,z)= x2+y2+z2-19=0

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