Cho \(\cos x = \dfrac{1}{2}...

B

- Hướng dẫn giải

Ta có: \(\cos x = \dfrac{1}{2} \Rightarrow {\cos ^2}x = \dfrac{1}{4}\).

Mà \({\sin ^2}x + {\cos ^2}x = 1\).

\( \Rightarrow {\sin ^2}x = 1 - {\cos ^2}x = 1 - \dfrac{1}{4} = \dfrac{3}{4}\)

Vậy \(P = 3{\sin ^2}x + 4{\cos ^2}x\)

\(= 3.\dfrac{3}{4} + 4.\dfrac{1}{4} = \dfrac{{13}}{4}\)