18. f is increasing on the open interval. f is decreasing on the open intervals and. 19. f is decreasing on…

Explanation:

Step1: Recall increasing - decreasing function rules

A function $y = f(x)$ is increasing when the graph goes up from left - to - right and decreasing when the graph goes down from left - to - right.

Step2: Analyze the first graph (question 18)

For the first graph, we observe that the function is increasing when $x\in(-1,1)$. It is decreasing when $x\in(-\infty,-1)$ and $x\in(1,\infty)$.

Step3: Analyze the second graph (question 19)

For the second graph, the function is decreasing when $x\in(-\infty,3)$ and $x\in(3,\infty)$.

Answer:

18. $f$ is increasing on the open interval $(-1,1)$. $f$ is decreasing on the open intervals $(-\infty,-1)$ and $(1,\infty)$.
19. $f$ is decreasing on the open intervals $(-\infty,3)$ and $(3,\infty)$.