Câu 2: 

Ta có: 

\(\begin{array}{l}
T = \dfrac{t}{n} = \dfrac{{120}}{{60}} = 2s\\
{t_1} = 8s = 4.2 = 4T \Rightarrow s = 4.4A = 16A\\
 \Rightarrow 64 = 16A \Rightarrow A = 4\left( {cm} \right)
\end{array}\)

Câu 3: 

a) Ta có: 

\(\begin{array}{l}
\lambda  = \dfrac{v}{f} = \dfrac{{10}}{{10}} = 1\left( {cm} \right)\\
{k_1} = \dfrac{{\dfrac{L}{2}}}{{\dfrac{\lambda }{2}}} = \dfrac{L}{\lambda } = \dfrac{{10}}{1} = 10\\
 \Rightarrow {N_1} = 2.9 + 1 = 19
\end{array}\)

b) Ta có: 

\(\begin{array}{l}
{k_2} = \dfrac{{\dfrac{L}{2} - \dfrac{\lambda }{4}}}{{\dfrac{\lambda }{2}}} = \dfrac{{L - \dfrac{\lambda }{2}}}{\lambda } = \dfrac{{10 - 0,5}}{1} = 9,5\\
 \Rightarrow {N_2} = 2.9 = 18
\end{array}\)