Câu 17: 

Ta có: 

\(\begin{array}{l}
\dfrac{T}{2} = {t_2} - {t_1} = 2,5 - 1,75 = 0,75 \Rightarrow T = 1,5\left( s \right)\\
v = \dfrac{{2A}}{{\dfrac{T}{2}}} = \dfrac{{2A}}{{0,75}} = 16 \Rightarrow A = 6\left( {cm} \right)\\
 \Rightarrow {x_0} = A = 6\left( {cm} \right)
\end{array}\)

Câu 18: 

Ta có: 

\(\begin{array}{l}
\cos {\varphi _1} = \dfrac{{{x_1}}}{A} = \dfrac{{2,5}}{5} = \dfrac{1}{2} \Rightarrow {\varphi _1} =  - \dfrac{\pi }{3}\left( {rad} \right)\\
\alpha  = \omega \Delta t = 10\pi .0,1 = \pi \left( {rad} \right)\\
 \Rightarrow {\varphi _2} = {\varphi _1} + \alpha  =  - \dfrac{\pi }{3} + \pi  = \dfrac{{2\pi }}{3}\left( {rad} \right)\\
{x_2} = A\cos {\varphi _2} = 5\cos \dfrac{{2\pi }}{3} =  - 2,5\left( {cm} \right)
\end{array}\)