Câu 1: 

Ta có: 

\(x = 5\cos \left( {4\pi t + \dfrac{\pi }{3}} \right) \Rightarrow \left\{ \begin{array}{l}
A = 5cm\\
{\varphi _0} = \dfrac{\pi }{3}\left( {rad} \right)
\end{array} \right.\)

Câu 2: 

Ta có: 

\(\begin{array}{l}
x =  - 4\cos \left( {4\pi t + \dfrac{\pi }{3}} \right) = 4\cos \left( {4\pi t - \dfrac{{2\pi }}{3}} \right)\\
 \Rightarrow \left\{ \begin{array}{l}
A = 4cm\\
\omega  = 4\pi \left( {rad/s} \right)\\
{\varphi _0} =  - \dfrac{{2\pi }}{3}\left( {rad} \right)
\end{array} \right.\\
T = \dfrac{{2\pi }}{\omega } = \dfrac{{2\pi }}{{4\pi }} = 0,5\left( s \right)\\
f = \dfrac{1}{T} = \dfrac{1}{{0,5}} = 2Hz
\end{array}\)

Câu 3: 

Ta có: 

\(\begin{array}{l}
\dfrac{\lambda }{2} = \dfrac{l}{{6 - 1}} = \dfrac{{100}}{5} = 20 \Rightarrow \lambda  = 40\left( {cm} \right)\\
v = \lambda f = 40.100 = 4000\left( {cm/s} \right)
\end{array}\)