Răspuns :
[tex]\displaystyle \\
E = cos \frac{\pi}{5} - cos \frac{3\pi}{5} \\ \\
\frac{\pi}{5} =\frac{180}{5} = 36^o ~; ~~36^o \ \textless \ 90^o~\Longrightarrow~unghiul ~ \frac{\pi}{5} ~este ~in~cadranul~1. \\ \\
\texttt{Cosinusul este pozitiv in cadranul 1. }\\ \\
\Longrightarrow~ cos \frac{\pi}{5} \ \textgreater \ 0\\ \\
\frac{3\pi}{5} =\frac{3\cdot 180}{5} =108^o ~; ~108^o \ \textgreater \ 90^o~\Longrightarrow~unghiul ~ \frac{3\pi}{5} ~e ~in~cadranul~2. \\ \\
\texttt{Cosinusul este negativ in cadranul 2. }
[/tex]
[tex]\displaystyle \\ \Longrightarrow~cos\frac{3\pi}{5} \ \textless \ 0~~ \Longrightarrow~~ -cos\frac{3\pi}{5} \ \textgreater \ 0 \\ \\ \Longrightarrow~E = cos \frac{\pi}{5} - cos \frac{3\pi}{5} = cos \frac{\pi}{5} +(- cos \frac{3\pi}{5}) \ \textgreater \ 0 \\ \\ \text{Expresia E este pozitiva deoarece ambii termmeni sunt pozitivi.}[/tex]
[tex]\displaystyle \\ \Longrightarrow~cos\frac{3\pi}{5} \ \textless \ 0~~ \Longrightarrow~~ -cos\frac{3\pi}{5} \ \textgreater \ 0 \\ \\ \Longrightarrow~E = cos \frac{\pi}{5} - cos \frac{3\pi}{5} = cos \frac{\pi}{5} +(- cos \frac{3\pi}{5}) \ \textgreater \ 0 \\ \\ \text{Expresia E este pozitiva deoarece ambii termmeni sunt pozitivi.}[/tex]
