$x=A.cos(ωt+\dfrac{\pi}{2})$
$+$ Có: $W_đ=W_t⇒n=1$
$⇔x=±\dfrac{A}{√2}⇒\begin{cases} \varphi=±\dfrac{\pi}{4}\\\varphi=±\dfrac{3\pi}{4}\ \end{cases}$
Vậy cứ mỗi $90^o$ thì $W_đ=W_t$
$+$ Vì: $\dfrac{\pi}{60}=90^o$
$⇔\dfrac{\pi}{60}=\dfrac{T}{4}⇒T=\dfrac{\pi}{15}$
BẠN THAM KHẢO.