`a=75pi^2.sin(5pit+pi/2)`

`=> a_(max)=75pi^2=omega^2.A=(5pi)^2.A`

`=>` Biên độ: `A=(a_(max))/((5pi)^2)=(75pi^2)/((5pi)^2)=3(cm)`

`+)` Có: `a=-omega^2.x`

`->`$x = -\frac{a}{\omega^2} = -\frac{75\pi^2 \sin(5\pi t + \frac{\pi}{2})}{(5\pi)^2} \\ = -\frac{75\pi^2 \sin(5\pi t + \frac{\pi}{2})}{25\pi^2} \\ = -\frac{75}{25} \sin(5\pi t + \frac{\pi}{2}) \\ = -3 \sin(5\pi t + \frac{\pi}{2}) \text{ (cm)}$

`->x=-3cos(5pit+pi)` (cm)

`->` PT dao động của vật là:

`x=3cos(5pit+pi)(cm)`

`->v=-A.omega.sin(5pit+pi)`

`=-3.5pi.sin(5pit+pi)`

`-` Tại `t=2/15` có:

`+)` $x= 3 \cos\left(5\pi \cdot \frac{2}{15} + \pi\right) \\= 3 \cos\left(\frac{2\pi}{3} + \pi\right) \\= 3 \cos\left(\frac{5\pi}{3}\right)  \\= 3 \cdot \frac{1}{2} \\= \frac{3}{2} \text{ (cm)}$

`+)` $v = -15\pi \sin\left(5\pi \cdot \frac{2}{15} + \pi\right) \\= -15\pi \sin\left(\frac{2\pi}{3} + \pi\right) \\= -15\pi \sin\left(\frac{5\pi}{3}\right) \\= -15\pi \cdot \left(-\frac{\sqrt{3}}{2}\right) \\= \frac{15\pi\sqrt{3}}{2} \text{ (cm/s)}$