$L=2A=8cm \Rightarrow A=4cm$
$\omega =\dfrac{2\pi}{T}=\dfrac{2\pi}{2}=\pi (rad/s)$
$t=0; x=0$ theo chiều âm
$\Rightarrow \varphi=\dfrac{\pi}{2}$
$a) PTDĐ: x=4cos(\pi t+\frac{\pi}{2})$
$b) v_{max}=\omega.A=4\pi (cm/s)$
$c) x=2=\dfrac{A}{2} \Rightarrow \alpha=(\pi t± \frac{\pi}{3})$
$x=-2=-\dfrac{A}{2} \Rightarrow \alpha=(\pi t± \frac{2\pi}{3})$