EC_308_Solutions_to_Problem Set 2_Fall_2023

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University of Alabama *

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308

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Economics

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Jan 9, 2024

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EC 308 Intermediate Microeconomics Problem Set 2 Solutions Problem 1: Bob’s utility function for soda and pizza is 𝑢(? 1 , ? 2 ) = ? 1 ? 2 where ? 1 is the number of cans of soda and ? 2 is the number of slices of pizza that Bob consumes. a) Graph Bob’s indifference curve, labeling at least three points. b) If given the choice between 2 slices of pizza and 4 cans of soda or 4 slices of pizza and 2 cans of soda would Bob prefer the bundle with more pizza, the bundle with more soda, or be indifferent between the two bundles? Bob’s utility from bundle (4 sodas, 2 pizza slices) is: 𝒖(?, ?) = √? √? = ?√? . Bob’s utility from bundle (2 sodas, 4 pizza slices) is 𝒖(?, ?) = √? √? = ?√? . Since Bob’s utility from both bundles is the same, Bob is indiff erent between them. c) Bob was consuming 2 slices of pizza and 4 cans of soda. Alice offered to give Bob 1 slice of pizza in exchange for 1 can of soda. Would Bob want to make this trade? Bob’s utility before the trade is 𝒖(?, ?) = √? √? = ?√? . If Bob makes the trade, he will consume 3 slices of pizza and 3 cans of soda. Bob’s utility after the trade would be 𝒖(?, ?) = √? √? = ? . Since Bob’s utility after the trade is greater than his util ity before the trade, Bob would want to make the trade.
d) Derive the formula for Bob’s marginal rate of substitution bet ween pizza and soda. Bob’s marginal rate of substitution (MRS) is given by: 𝑴?? = − 𝑴𝑼 ? 𝑴𝑼 ? 𝑴𝑼 ? is Bob’s marginal utility from soda. It is the partial derivative of Bob’s utility functi on with respect to changes in the consumption of soda (holding Bob’s consumption of pizza constant). That is, using the ‘power rule’ for derivatives (i.e, the derivative of ? 𝒌 is 𝒌? 𝒌−? ), Bob’s marginal utility from Soda is:: 𝑴𝑼 ? = ?. ?? ? ?.? ? ? ?.? = ?. ? ? ? ? ? Similarly, 𝑴𝑼 ? = ?.? ? ? ? ? . Hence, Bob’s MRS is: 𝑴?? = − 𝑴𝑼 ? 𝑴𝑼 ? = − ?. ? ? ? ? ? ?. ? ? ? ? ? = − ( ?. ? ? ? ? ? ) ( ? ? ?. ? ? ? ) = −? ? /? ? Problem 2: Alice’s preferences over apples and oranges are represented by utility function: 𝑢(?, ?) = 10? 0.5 ? 0.5 where ? is the number of apples and ? is the number of oranges she consumes. a) What is the relationship between a consumer’s marginal rate of substitution and the consumer’s indifference curve? The marginal rate of substitution is the slope of the consumer’s indifference curve. b) Write a general formula for Alice’s marginal rate of substitution (MRS) between apples and oranges. Alice’s marginal rate of substitution (MRS) is given by: 𝑴?? = − 𝑴𝑼 ? 𝑴𝑼 ? 𝑴𝑼 ? = ?√? √? , 𝑴𝑼 ? = ?√? √? . Hence, Alice ’s MRS is: 𝑴?? = − 𝑴𝑼 ? 𝑴𝑼 ? = − ( ?√? √? ) ( √? ?√? ) = − ? ? . c) What is Alice’s marginal rate of substitution (MRS) when she is consuming bundle (25,5) ?
Her MRS is ? ?? = − ? ? . d) What is Alice’s marginal rate of substitution when she is consuming bundle (25,25) ? Her MRS is ?? ?? = −? . Problem 3: Suppose the price of good 1 is 𝑝 1 = $5 , the price of good 2 is 𝑝 2 = $2 , and four consumers, Alice, Bob, Charlotte, and David, each have 𝑚 = $100 available to spend on these two goods. a) Alice has utility function 𝑢(? 1 , ? 2 ) = ? 1 ? 2 . Find Alice’s demand functions for her optimal quantities to consume of goods 1 and 2 (i.e., find formulas for her optimal values of ? 1 and ? 2 ). Given the prices and the money she has available, what is Alice’s optim al consumption of goods 1 and 2? We solve the utility maximization problem for utility function U(x 1, x 2 ) = ? ? ? ? = (x 1 1/2 )(x 2 1/2 ). Step 1: Write down the budget constraint: p 1 x 1 + p 2 x 2 = m. Step 2: Write down the optimality condition: MRS = - MU 1 /MU 2 = - p 1 /p 2 . Step 3: Find MU 1 and MU 2 which are the partial derivatives of U(x 1, x 2 ) with respect to x 1 and x 2 : MU 1 = 0.5x 1 -1/2 x 2 1/2 MU 2 = 0.5x 1 1/2 x 2 -1/2 Recall from the first day of class that the rules of exponents imply: ? ? ? ? = ? ?−? . So MU 1 /MU 2 = 𝑴𝑼 ? 𝑴𝑼 ? = ?.?? ? −?.? ? ? ?.? ?.?? ? ?.? ? ? −?.? = ( ?.? ?.? ) (? ? −?.?−?.? )(? ? ?.?−(−?.?) ) = ? ? −? ? ? ?.?+?.? = ? ? ? ? . So the optimality condition is: x 2 /x 1 = p 1 /p 2 . So x 2 = x 1 p 1 /p 2 = x 1 p 1 /p 2 Step 4: Substitute the above formula for x 2 into the budget constraint: p 1 x 1 + p 2 x 2 = p 1 x 1 + p 2 (x 1 p 1 /p 2 ) = p 1 x 1 + p 1 x 1 = 2p 1 x 1 = m. Step 5: Solve for x 1 : x 1 = m/2p 1. Step 6: Substitute the solution for x 1 into the formula for x 2 and solve for x 2 : ? ? = ? ? 𝒑 ? 𝒑 ? = ( 𝒎 ?𝒑 ? ) ( 𝒑 ? 𝒑 ? ) = 𝒎 ?𝒑 ? Step 7: Substitute the given values into the formulas for x 1 and x 2 : p 1 = $5, p 2 = $2, m = $100. So x 1 = m/2p 1 = 100/10 = 10 and x 2 = m/2p 2 = 100/4 = 25. The optimal bundle is (x 1 ,x 2 ) = (10, 25).
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