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$18$. $C(x)=0.04 x^{2}$$\newline$Over what interval is each function increasing and over what interval is each function decreasing?$\newline$SEE EXAMPLE $3$$\newline$$20$.$\newline$\begin{tabular}{|c|c|c|}$\newline$\hline$x$ & $f(x)=-0.3 x^{2}$ & $(x, y)$ \\$\newline$\hline$-2$ & $-1$.$2$ & $(-2,-1.2)$ \\$\newline$\hline$-1$ & $-0$.$3$ & $(-1,-0.3)$ \\$\newline$\hline $0$ & $0$ & $(0,0)$ \\$\newline$\hline $1$ & $-0$.$3$ & $(1,-0.3)$ \\$\newline$\hline $2$ & $-1$.$2$ & $(2,-1.2)$ \\$\newline$\hline$\newline$\end{tabular}