How can we use the method of Lagrange multipliers to extremize the function $f(x,y)=6x+8y$ subject to the constraint $g(x,y)=x^2+y^2-25=0$ ?
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Using the definitions from Wikipedia rather literally:
Clear[f, g]
f[x_, y_] := 6 x + 8 y
g[x_, y_] := x^2 + y^2 - 25
Solve[
D[f[x, y] - lambda g[x, y], {{x, y, lambda}}] == 0,
{x, y}, lambda
]
(* Out: {{x -> -3, y -> -4}, {x -> 3, y -> 4}} *)
Unpacking the code, we start with defining the function to be optimized, and the constraint:
Clear[f, g]
f[x_, y_] := 6 x + 8 y
g[x_, y_] := x^2 + y^2 - 25
The method of Lagrange multipliers assures us that the extrema of the original function $f$ are stationary points for $\Lambda = f-\lambda \ g$. In order to find those, we calculate the gradient of this auxiliary function $\Lambda$, and set it to zero:
D[f[x, y] - lambda g[x, y], {{x, y, lambda}}] == 0
For a function $f$ of $n$ variables, this gives us a set of $n+1$ equations in $n+1$ variables, the original ones plus the multiplier, which we can solve using Solve.