Cho tam giác ABC. Tìm giá trị lớn nhất của biểu thức P

\(P = \sin \frac{A}{2}\sin \frac{B}{2}\sin \frac{C}{2} = \frac{1}{2}\left[ {\cos \left( {\frac{A}{2} - \frac{B}{2}} \right) - \cos \left( {\frac{A}{2} + \frac{B}{2}} \right)} \right]\sin \frac{C}{2} = \frac{1}{2}\left[ {\cos \left( {\frac{A}{2} - \frac{B}{2}} \right) - \sin \frac{C}{2}} \right]\sin \frac{C}{2}\)

\( = - \frac{1}{2}{\sin ^2}\frac{C}{2} + \frac{1}{2}\cos \left( {\frac{A}{2} - \frac{B}{2}} \right).\sin \frac{C}{2}\)\( = - \frac{1}{2}\left[ {{{\sin }^2}\frac{C}{2} - \cos \left( {\frac{A}{2} - \frac{B}{2}} \right).\sin \frac{C}{2} + \frac{1}{4}{{\cos }^2}\left( {\frac{A}{2} - \frac{B}{2}} \right)} \right] + \frac{1}{8}{\cos ^2}\left( {\frac{A}{2} - \frac{B}{2}} \right)\)\( = - \frac{1}{2}{\left[ {\sin \frac{C}{2} - \frac{1}{2}\cos \left( {\frac{A}{2} - \frac{B}{2}} \right)} \right]^2} + \frac{1}{8}{\cos ^2}\left( {\frac{A}{2} - \frac{B}{2}} \right) \le \frac{1}{8}{\cos ^2}\left( {\frac{A}{2} - \frac{B}{2}} \right) \le \frac{1}{8}\).

Dấu \( = \) xảy ra \( \Leftrightarrow \left\{ \begin{array}{l}\sin \frac{C}{2} - \frac{1}{2}\cos \left( {\frac{A}{2} - \frac{B}{2}} \right) = 0\\{\cos ^2}\left( {\frac{A}{2} - \frac{B}{2}} \right) = 1\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}\sin \frac{C}{2} = \frac{1}{2}\\A = B\end{array} \right. \Leftrightarrow A = B = C = 60^\circ \).

Vậy biểu thức \(P\) đạt giá trị lớn nhất bằng \(\frac{1}{8}\) khi và chỉ khi tam giác \(ABC\) đều.