Giải thích các bước giải:
Ta có:
\(\begin{array}{l}
\frac{T}{2} < \alpha < T \Leftrightarrow \pi < \alpha < 2\pi \Rightarrow \sin \alpha < 0\\
{\sin ^2}\alpha + {\cos ^2}\alpha = 1\\
\sin \alpha < 0 \Rightarrow sin\alpha = - \sqrt {1 - {{\cos }^2}\alpha } = - \sqrt {1 - {{\left( {\frac{3}{5}} \right)}^2}} = - \frac{4}{5}\\
\cos \left( {\alpha - \frac{T}{3}} \right) = \cos \left( {\alpha - \frac{{2\pi }}{3}} \right) = \cos \alpha .\cos \frac{{2\pi }}{3} + \sin \alpha .\sin \frac{{2\pi }}{3} = \frac{3}{5}.\left( { - \frac{1}{2}} \right) + \frac{{\sqrt 3 }}{2}.\left( { - \frac{4}{5}} \right) = \frac{{ - 3 - 4\sqrt 3 }}{{10}}
\end{array}\)
