Biết \(\cos \alpha = \frac...

C

- Hướng dẫn giải

\({\rm{cos}}\alpha = \frac{1}{3}\\ \Rightarrow P = {\sin ^2}\alpha + 3c{\rm{o}}{{\rm{s}}^2}\alpha \\= \left( {{{\sin }^2}\alpha + {\rm{co}}{{\rm{s}}^2}\alpha } \right) + 2{\rm{co}}{{\rm{s}}^2}\alpha \\= 1 + 2{\rm{co}}{{\rm{s}}^2}\alpha = \dfrac{{11}}{9}\)