The Monthly Mortgage Payment in Dollars, P, for a House

Essay

The monthly mortgage payment in dollars, P, for a house is a function of three variables P = f(A, r, N), where A is the amount borrowed in dollars, r is the interest rate, and N is the number of years before the mortgage is paid off.It is given that: $$\begin{array} { l l l } f ( 100000,7,20 ) = 775.29 , & f ( 100000,8,20 ) = 836.44 , & f ( 100000,7,25 ) = 706.77 \\f ( 120000,7,20 ) = 930.35 , & f ( 120000,8,20 ) = 1003.72 , & f ( 120000,7,25 ) = 848.13\end{array}$$ Estimate the value of $$\left. \frac { \partial f } { \partial r } \right| _ { ( 100000,7,20 ) }$$