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`:
$$ MRS = - \frac{MU_X \left( X, Y \right)}{MU_Y \left( X, Y \right)} = - \frac{Y}{X} $$Question
Alice's utility function is `u (X, Y) = X^{234} Y^{53}`.
What is the Marginal Rate of Substitution of strawberries for a chocolate in function of X and Y?
$$
MU_X = \frac{du(X, Y)}{dX} = 234 X^{234 - 1} Y^{53} = 234 X^{233} Y^{53}
$$
$$
MU_Y = \frac{du(X,Y)}{dY} = 53 X^{234} Y^{53 - 1} = 53 X^{234} Y^{52}
$$
Therefore
$$ MRS = - \frac{MU_X \left( X, Y \right)}{MU_Y \left( X, Y \right)} = - \frac{234 X^{233} Y^{53}}{53 X^{234} Y^{52}} = - \frac{234 Y}{53 X} $$