where the derivative formula_59 measures how responsive formula_41 is to changes in formula_44. The derivative can be computed if we know the rate law for formula_41. For example, if we assume that the rate law is formula_63 then the derivative is formula_64. We can also use a similar strategy to compute the change in formula_40 as a result of the change formula_47. This time the change in formula_40 is a result of two changes, the change in formula_42 itself and the change in formula_44. We can express these changes by summing the two individual contributions: