4. (15%)Suppose the utility function for goods $x$ and $y$ is given by

$$U(x, y) = xy + y.$$

a. Let \( P_{x} \) and \( P_{y} \) be the price of \( x \) and \( y \). Calculate the expenditure function for \( x \) and \( y \).
b. Compute the compensated demand functions for goods \(x\) and \(y\).

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