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^{960} Y^{50}`.
What is the Marginal Rate of Substitution of strawberries for a chocolate in function of X and Y?
$$
MU_X = \frac{du(X, Y)}{dX} = 960 X^{960 - 1} Y^{50} = 960 X^{959} Y^{50}
$$
$$
MU_Y = \frac{du(X,Y)}{dY} = 50 X^{960} Y^{50 - 1} = 50 X^{960} Y^{49}
$$
Therefore
$$ MRS = - \frac{MU_X \left( X, Y \right)}{MU_Y \left( X, Y \right)} = - \frac{960 X^{959} Y^{50}}{50 X^{960} Y^{49}} = - \frac{960 Y}{50 X} $$