`~rai~`
\(x=12.\cos\left(\dfrac{\pi}{2}t-\dfrac{\pi}{3}\right)\\\Rightarrow x'=12.\left[\cos\left(\dfrac{\pi}{2}t-\dfrac{\pi}{3}\right)\right]'.\left(\dfrac{\pi}{2}t-\dfrac{\pi}{3}\right)'\\\quad\quad=12.\left[-\sin\left(\dfrac{\pi}{2}t-\dfrac{\pi}{3}\right)\right].\dfrac{\pi}{2}\\\quad\quad=6\pi.\cos\left(\dfrac{\pi}{2}-\dfrac{\pi}{3}+\dfrac{\pi}{2}\right)\\\quad\quad=6\pi.\cos\left(\dfrac{\pi}{2}t+\dfrac{\pi}{6}\right).\\\text{Áp dụng công thức:}\\+)(\cos u)'=-\sin u.u'\\+)x'=1\\+)c'=0\\+)\cos\left(x+\dfrac{\pi}{2}\right)=-\sin x.\)
