Example
Alice's utility function is `u \left( X, Y \right) = XY`.
Her marginal rate of substitution is the ratio of the marginal utilities `MU_X` and `MU_Y`: $$ MRS = - \frac{MU_X}{MU_Y} = - \frac{Y}{X} $$
Question
Alice's utility function is `u (X, Y) = X^{652} Y^{157}`.
What is her Marginal Rate of Substitution?
$$ MU_X = \frac{du(X, Y)}{dX} = 652 X^{652 - 1} Y^{157} = 652 X^{651} Y^{157} $$
$$ MU_Y = \frac{du(X,Y)}{dY} = 157 X^{652} Y^{157 - 1} = 157 X^{652} Y^{156} $$
Therefore $$ MRS = - \frac{MU_X}{MU_Y} = - \frac{652 X^{651} Y^{157}}{157 X^{652} Y^{156}} = - \frac{652 Y}{157 X} $$