Week 8: Partial derivatives with constrained variables, Taylor's formula for two variables

Thomas Calculus, 11th edition, pages: 1049 - 1060

Sec 14.9 Partial Derivatives with Constrained Variables

In finding partial derivatives of functions like w = f(x,y), we have assumed x and y to be independent. In many applications, however, this is not the case. For example, the internal energy U of a gas may be expressed as a function  U= f (P, V, T) of pressure P, volume V, and temperature T. If the individual molecules of the gas do not interact, however, P, V, and T obey (and are constrained by) the ideal gas law

PV = nRT (n and R constant),

and fail to be independent. In this section we learn how to find partial derivatives in situations
like this, which you may encounter in studying economics, engineering, or physics.

Sec 14.10 Talor's Formula for Two Variables

This section uses Taylor’s formula to derive the Second Derivative Test for local extreme values (Section 14.7) and the error formula for linearizations of functions of two independent variables (Section 14.6). The use of Taylor’s formula in these derivations leads to an extension of the formula that provides polynomial approximations of all orders for functions of two independent variables.