Relate graph direction and instantaneous rate
The segment through P and Q is the tangent to the smooth curve at the stated input
\[f(x)=-x^{2}-1\]
\[x=0,\quad P=(-\frac{1}{2},-1),\quad Q=(\frac{1}{2},-1)\]
The segment through P and Q is the tangent to the smooth curve at the stated input