- #1
arcyqwerty
- 6
- 0
Homework Statement
Find max/min of x^2+y^2+z^2 given x^4+y^4+z^4=3
Homework Equations
Use of gradient vectors related by LaGrange Multiplier
The Attempt at a Solution
[tex]\begin{gathered}
f\left( {x,y,z} \right) = {x^2} + {y^2} + {z^2};g\left( {x,y,z} \right) = {x^4} + {y^4} + {z^4} - 3 = 0 \\
\vec \nabla f = \left\langle {2x,2y,2z} \right\rangle ;\vec \nabla g = \left\langle {4{x^3},4{y^3},4{z^3}} \right\rangle \\
\left\langle {2x,2y,2z} \right\rangle = \lambda \left\langle {4{x^3},4{y^3},4{z^3}} \right\rangle \\
2{x^2} = 2{y^2} = 2{z^2} \to x = \pm y = \pm z \\
3{x^4} - 3 = 0 \to {x^4} = 1 \to x = \pm 1 \to y = \pm 1,z = \pm 1 \\
\max = f\left( {1,1,1} \right) = f\left( {1,1, - 1} \right) = f\left( {1, - 1,1} \right) = f\left( {1, - 1, - 1} \right) = \\
f\left( { - 1,1,1} \right) = f\left( { - 1,1, - 1} \right) = f\left( { - 1, - 1,1} \right) = f\left( { - 1, - 1, - 1} \right) = 3 \\
\end{gathered}[/tex]
So I found the maximum but does the minimum exist?