Tính f ′′ ( π/ 24 ) .

Ta có \(f'\left( x \right) = - 2\cos \left( {2x + \frac{\pi }{{12}}} \right)\); \(f''\left( x \right) = 4\sin \left( {2x + \frac{\pi }{{12}}} \right)\).

Do đó \(f''\left( {\frac{\pi }{{24}}} \right) = 4\sin \left( {2.\frac{\pi }{{24}} + \frac{\pi }{{12}}} \right) = 2\).

Trả lời: 2.