A fence must be built to enclose a rectangular area of \( 20,000 \mathrm{ft}^{2} \). Fencing material costs \( \$ 2 \) per foot for the two sides facing north and south and \( \$ 4 \) per foot for the other two sides. Find the cost of the least expensive fence. The cost of the least expensive fence is \( \$ \square \). (Simplify your answer.)

Find the intervals on which \( f \) is increasing and the intervals on which it is decreasing. \( f(x)=-x^{3}-9 x \) Aelect the correct choice and, if necessary, fill in the answer box(es) to complete your choice. A. The function is increasing on the open interval(s) \( \square \) and decreasing on the open interval(s) (Simplify your answers. Type your answers in interval notation. Use a comma to separate answers as needed.) B. The function is increasing on the open interval(s) (Simplify your answer. Type your answer in interval notation. Use a comma to separate answers as needed.) C. The function is decreashing on the open interval(s) \( \square \). The function is never increasing. (Simplify your answer. Type your answer in interval notation. Use a comma to separate answers as needed.) D. The function is never increasing nor decreasing.

Montrer que \( \overrightarrow{A^{\prime} B^{\prime}}=k \overrightarrow{A^{\prime} E^{\prime}} \), déchire que \( \overrightarrow{A^{\prime} B^{\prime}}=k \overrightarrow{C^{\prime} D^{\prime}} \)

18. Nustatykite, kuriame koordinatiniame ketvirtyje yra šie kampai: a) \( 185^{\circ} ; 480^{\circ} ;-714^{\circ} ;-2002^{\circ} \); b) \( \frac{19 \pi}{7} ; 2,01 \pi ;-\frac{49 \pi}{4} ;-3,2 \)

8. Observe the figures below.

Montren que: \( \frac{\left(9^{n+1}+9^{n}\right)^{8}}{\left(3^{2 n}-3^{2 n+1}\right)^{2}} \in \mathbb{Z} \)