estimate derivatives using the symmetric difference quotient $(S D Q),$ defined as the average of the difference quotients at $h$ and $-h :$
$$\begin{array}{r}{\frac{1}{2}\left(\frac{f(a+h)-f(a)}{h}+\frac{f(a-h)-f(a)}{-h}\right)} \\ {=\frac{f(a+h)-f(a-h)}{2 h}}\end{array}$$
The SDQ usually gives a better approximation to the derivative than the difference quotient.
In Exercises $69-70,$ traffic speed $S$ along a certain road (in kilometers per hour) varies as a function of traffic density $q$ (number of cars per kilometer of road). Use the following data to answer the questions:
\begin{equation}
\begin{array}{l}{\text { Estimate } S^{\prime}(80)} \end{array}
\end{equation}