$\omega=10(rad/s)$
$a=-\omega^2x\to x=\dfrac{-2}{-10^2}=0,02(m)=2(cm)$
$A=4(cm)\to x=\dfrac{A}{2}$
$x_o=4\cos\dfrac{\pi}{2}=0$
Đi từ $x_o=0$ đến $x=\dfrac{A}{2}$ mất thời gian là $t=\dfrac{T}{12}(s)$
$T=\dfrac{2\pi}{\omega}=\dfrac{\pi}{5}(s)$
$\to t=\dfrac{\pi}{5}.\dfrac{1}{12}=\dfrac{\pi}{60}(s)$