4. Theory of the Consumer (18 points) Why would we expect a rational consumer, who consumes positive quantities of two goods X and y, to equate the marginal rate of substitution between them with the relevant _ PX price ratio (i.e., Msy ) ? In the process of answering the question, you should Pa dene the marginal rate of substitution both in words and mathematically (hint: think about moving along an indifference curve). Then relate the mathematical expression to one obtained from the solution of the consumer's problem of maximizing utility subject to a budget constraint