Đáp án: B
Giải thích các bước giải:
$\begin{array}{l}
108)\\
f\left( x \right) = cos3x.cosx\\
f\left( x \right) = cos\left( {\frac{{{\rm{4x + 2x}}}}{2}} \right).c{\rm{os}}\left( {\frac{{4x - 2x}}{2}} \right)\\
f\left( x \right) = \frac{1}{2}.\left( {{\rm{cos4x + cos2x}}} \right)\\
\Rightarrow \int {f\left( x \right)dx} = \int {\frac{1}{2}\left( {cos4x + cos2x} \right)} dx\\
= \frac{1}{2}\int {\frac{1}{4}cos4x\left( {d4x} \right) + \frac{1}{2}\int {\frac{1}{2}.cos2xd\left( {2x} \right)} } \\
= \frac{1}{8}.\sin 4x + \frac{1}{4}.\sin 2x
\end{array}$
