Bài tập 5.100 trang 215 SBT Toán 11

Ta có:

\(\begin{array}{l}
y = \frac{1}{2}\left( {\cos x - \cos 3x} \right).\sin 3x = \frac{1}{2}\cos x\sin 3x - \frac{1}{2}\cos 3x\sin 3x\\
 = \frac{1}{4}\left( {\sin 2x + \sin 4x} \right) - \frac{1}{4}\sin 6x = \frac{1}{4}\sin 2x + \frac{1}{4}\sin 4x - \frac{1}{4}\sin 6x
\end{array}\)

\(\begin{array}{l}
y' = \frac{1}{2}\cos 2x + \cos 4x - \frac{3}{2}\cos 6x\\
y'' =  - \sin 2x - 4\sin 4x + 9\sin 6x
\end{array}\)

-- Mod Toán 11 HỌC247