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^{814} Y^{690}`.
What is her Marginal Rate of Substitution?
$$ MU_X = \frac{du(X, Y)}{dX} = 814 X^{814 - 1} Y^{690} = 814 X^{813} Y^{690} $$
$$ MU_Y = \frac{du(X,Y)}{dY} = 690 X^{814} Y^{690 - 1} = 690 X^{814} Y^{689} $$
Therefore $$ MRS = - \frac{MU_X}{MU_Y} = - \frac{814 X^{813} Y^{690}}{690 X^{814} Y^{689}} = - \frac{814 Y}{690 X} $$