Đáp án:

 \({W_d} = \dfrac{1}{4}m{\omega ^2}{A^2} + \dfrac{1}{4}m{\omega ^2}{A^2}\cos \left( {2\omega t + \varphi } \right)\)

Giải thích các bước giải:

Ta có: 

\(\begin{array}{l}
{W_d} = \dfrac{1}{2}m{\omega ^2}{x^2} = \dfrac{1}{2}m{\omega ^2}{A^2}{\cos ^2}\left( {\omega t + \varphi } \right)\\
 \Rightarrow {W_d} = \dfrac{1}{2}m{\omega ^2}{A^2}\left( {\dfrac{{1 + \cos \left( {2\omega t + \varphi } \right)}}{2}} \right)\\
 \Rightarrow {W_d} = \dfrac{1}{4}m{\omega ^2}{A^2} + \dfrac{1}{4}m{\omega ^2}{A^2}\cos \left( {2\omega t + \varphi } \right)
\end{array}\)