Tìm GTLN (max), GTNN (min) của F=81^cos^2X min=2 , max=81

* Hướng dẫn giải

$$\begin{gathered} \mathrm{F}\text{'}=1-\frac{81}{{\mathrm{t}}^2}=\frac{{\mathrm{t}}^2-81}{{\mathrm{t}}^2} \\ \mathrm{F}\text{'}=0\iff {\mathrm{t}}^2-81=0\iff \mathrm{t}=9 ( \mathrm{t}=-9 \mathrm{bị} \mathrm{loại} ) \\ \mathrm{Ta} \mathrm{có} \mathrm{F}\left(1\right)=82; \mathrm{F}\left(9\right)=18; \mathrm{F}\left(81\right)=82 \\ \Rightarrow \min \mathrm{F} =18 \mathrm{và} \max \mathrm{F}=82 \end{gathered}$$