## Calculus (3rd Edition)

The symmetric difference quotient is the slope of the secant line connecting the points $(a − h, f (a − h))$ and $(a + h, f (a + h))$ on the graph of $f$.
The symmetric difference quotient $$\frac{f(a+h)-f(a-h)}{2 h}$$ is the slope of the secant line connecting the points $(a − h, f (a − h))$ and $(a + h, f (a + h))$ on the graph of $f$. The difference in the function values is divided by the difference in the $x-$values.