Tập xác định $D=\mathbb{R}$
$\forall\, x\in D\Rightarrow -x\in D$
$\begin{array}{l}
f\left( x \right) = {\sin ^5}3x\cos \left( {3\pi - 7x} \right) = {\sin ^5}3x\cos \left( {\pi - 7x} \right)\\
= - {\sin ^5}3x\cos 7x\\
f\left( { - x} \right) = {\sin ^5}\left( { - 3x} \right)\cos \left( {3\pi - 7\left( { - x} \right)} \right)\\
= - {\sin ^5}3x\cos \left( {3\pi + 7x} \right)\\
= - {\sin ^5}3x\cos \left( {7x + \pi } \right)\\
= - {\sin ^5}3x - \left( {\cos 7x} \right)\\
= {\sin ^5}3x.\cos 7x = - f\left( x \right)
\end{array}$
Hàm lẻ.
