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Question

Solve using Lagrange multipliers. Find a vector in 3-space whose length is 5 and whose components have the largest possible sum.

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Answered 1 year ago
Answered 1 year ago

Let v=x,y,z\textbf{v}=\langle x,y,z\rangle be a vector in R3\mathcal{R}^3 with lenght v=5||\textbf{v}||=5. This is

x2+y2+z2=5x2+y2+z2=25\begin{aligned} \sqrt{x^2+y^2+z^2}=5\equiv x^2+y^2+z^2=25 \end{aligned}

Let f(x,y,z)=x+y+zf(x,y,z)=x+y+z the sum of the vector components. We need to maximize ff under the constraint x2+y2+z2=25x ^ { 2 } + y ^ { 2 } + z ^ { 2 } = 25. Using Lagrange Multipliers

f=λg1,1,1=λ2x,2y,2zλ=12x=12y=12zx=y=z\begin{aligned} \nabla f=\lambda \nabla g\Rightarrow \langle 1,1,1 \rangle = \lambda \langle 2 x , 2 y , 2 z \rangle\\ \lambda = \frac { 1 } { 2 x } = \frac { 1 } { 2 y } = \frac { 1 } { 2 z } \Rightarrow x = y = z \end{aligned}

Using this in the given constraint:

x2+y2+z2=253x2=25x=y=z=±533\begin{aligned} x ^ { 2 } + y ^ { 2 } + z ^ { 2 } = 25 \Rightarrow 3 x ^ { 2 } = 25 \Rightarrow x = y=z=\pm \frac { 5 \sqrt { 3 } } { 3 } \end{aligned}

We note that for x=y=z=533x=y=z=\frac{5\sqrt{3}}{3}, we obtain the largest component sum

S=3533=53\begin{aligned} S=3\frac{5\sqrt{3}}{3}=5\sqrt{3} \end{aligned}

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