A consumer spends an amount 𝑚𝑚 to buy 𝑥𝑥 units of one good at the price of 6 per unit and 𝑥𝑥 units of a different good at the price of 10 per unit. Here 𝑚𝑚 is positive and suppose that 8 < 𝑚𝑚 < 40. The consumer’s utility function is 𝑈(𝑥, y) = 𝑥y + y^2+ 2𝑥 + 2y, so that her problem is: Maximize 𝑥y+y^2 +2x+2y subject to 6𝑥+10𝑥y=𝑚 (a) Find the optimal quantities 𝑥∗ and y∗ and the Lagrange multiplier, all of them as functions of 𝑚. (b) Write down the maximum value of the utility function as a function of m. (c) Find the derivative of the above utility function for m=20 (d) What are the solutions for 𝑥∗ and y∗ if (i) 𝑚 ≤ 8? (ii) 𝑚 ≥ 40?
Question
A consumer spends an amount 𝑚𝑚 to buy 𝑥𝑥 units of one good at the price of 6 per unit and 𝑥𝑥 units of a different good at the price of 10 per unit. Here 𝑚𝑚 is positive and suppose that 8 < 𝑚𝑚 < 40. The consumer’s utility function is 𝑈(𝑥, y) = 𝑥y + y^2+ 2𝑥 + 2y, so that her problem is: Maximize 𝑥y+y^2 +2x+2y subject to 6𝑥+10𝑥y=𝑚 (a) Find the optimal quantities 𝑥∗ and y∗ and the Lagrange multiplier, all of them as functions of 𝑚. (b) Write down the maximum value of the utility function as a function of m. (c) Find the derivative of the above utility function for m=20 (d) What are the solutions for 𝑥∗ and y∗ if (i) 𝑚 ≤ 8? (ii) 𝑚 ≥ 40?
Solution
This is a problem of optimization with constraints, which can be solved using the method of Lagrange multipliers.
(a) The Lagrangian for this problem is:
L = x*y + y^2 + 2x + 2y + λ(m - 6x - 10xy)
where λ is the Lagrange multiplier. The first order conditions are:
∂L/∂x = y - 6λ - 10λy = 0 ∂L/∂y = x + 2y + 2 - 10λx = 0 ∂L/∂λ = m - 6x - 10xy = 0
Solving these equations simultaneously for x, y, and λ will give the optimal quantities x* and y* and the Lagrange multiplier as functions of m.
(b) Substituting x* and y* into the utility function U(x, y) = x*y + y^2 + 2x + 2y will give the maximum value of the utility function as a function of m.
(c) The derivative of the utility function with respect to m at m=20 can be found by differentiating the utility function with respect to m and then substituting m=20.
(d) If m ≤ 8, the constraint 6x + 10xy ≤ m becomes binding, and the consumer will choose x* and y* to satisfy this constraint. If m ≥ 40, the constraint is not binding, and the consumer will choose x* and y* to maximize the utility function without regard to the constraint.
Please note that this is a general approach to solve the problem. The actual solutions will depend on the specific form of the utility function and the constraint.
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