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Let T(x,y,z) = x^2 + y^2 + z^2 and h(x,y,z) = 2x + 3y - 5z + 4. Which one of the following systems of equations represents the Lagrange multiplier condition that must be satisfied by a point that maximises or minimises T subject to h(x,y,z) = 0?

Question

Let T(x,y,z) = x^2 + y^2 + z^2 and h(x,y,z) = 2x + 3y - 5z + 4. Which one of the following systems of equations represents the Lagrange multiplier condition that must be satisfied by a point that maximises or minimises T subject to h(x,y,z) = 0?

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Solution

The Lagrange multiplier condition is given by the equation ∇f = λ∇g, where f is the function to be maximized or minimized, g is the constraint function, and λ is the Lagrange multiplier.

In this case, f(x,y,z) = T(x,y,z) = x^2 + y^2 + z^2 and g(x,y,z) = h(x,y,z) = 2x + 3y - 5z + 4.

The gradient of f, ∇f, is given by the vector (2x, 2y, 2z) and the gradient of g, ∇g, is given by the vector (2, 3, -5).

Setting ∇f = λ∇g gives the system of equations:

2x = λ2, 2y = λ3, 2z = λ*(-5).

This simplifies to:

x = λ, y = 1.5λ, z = -2.5λ.

The constraint h(x,y,z) = 0 gives the equation 2x + 3y - 5z + 4 = 0. Substituting the expressions for x, y, and z from the system of equations gives:

2λ + 4.5λ + 12.5λ + 4 = 0, which simplifies to 19λ + 4 = 0.

Solving for λ gives λ = -4/19.

Substituting λ = -4/19 into the system of equations gives the point (x, y, z) = (-4/19, -6/19, 10/19) that maximizes or minimizes T subject to h(x,y,z) = 0.

So, the system of equations representing the Lagrange multiplier condition is:

x = λ, y = 1.5λ, z = -2.5λ, 19λ + 4 = 0.

This problem has been solved

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