Simple yet interesting applications of Calculus or Linear Algebra to Economics This is essentially a vast generalization of my previous question: Examples of separable ordinary differential equations in economics
I'm giving a talk to college-level math teachers on some applications of mathematics to Economics. My main goal is to try to convince them is that it's possible to integrate examples and methods from economics that are both elegant and lead to real economic insight into Calculus and Linear Algebra courses. I'm probably going to skip the Leontief input-output model, as well as very basic game theory and present- and future-value calculations, since I think those are all pretty standard. I want to keep the scope of the talk manageable, so I'm going to steer clear of any statistical methods.
I'm definitely going to talk about the Keynesian multiplier, the interpretation of Lagrange multipliers as shadow prices, and about how one can use implicit differentiation to determine the incidence of taxes on supplier and consumer.
My question is the following: does anyone have any suggestions for other topics?
 A: My personal favourite example is bond duration, which also gives insight into the meaning of a derivative.
The derivative of a function $f$ measures the sensitivity of $f(x)$ to a change in its input $x$. Functions whose derivatives are smaller are closer to being constant, so changing $x$ doesn't change $f(x)$ much, and vice versa. 
The price of a bond is a function of its yield. The modified duration of a bond whose present value is $1$ is (minus) the first derivative of the bond price.  It's one of the most important measures for bonds, because it tells you just what you would like to know when you buy a bond- namely how much interest rate risk you are taking on by buying it. 
A: One possible application of linear algebra to economics, which has been well established is its applicability to labour costs, and the production cost function. The input output function uses a linear algebra formulation, and the production functions' elasticity with respect labour is well studied. Profit models follow from these formulations, and have follow well established economic principles. The number of variables, in the production function, and so on depending upon the model, determine the production functions' elasticity.  
