a "first-order" system typically describes linear relationships, while a "second-order" system often involves nonlinear relationships.
The first blank can be filled with "second" to complete the phrase: "Second order involves nonlinear proportionality."
The second blank can be filled with "first" to complete the phrase: "First order involves linear proportionality."
In general, when discussing differential equations or systems, a "first-order" system typically describes linear relationships, while a "second-order" system often involves nonlinear relationships.