Làm giùm mình 3 câu này vs thanks mn nhiều

Câu 6:

\(T=\dfrac{2\pi}{\omega}=\dfrac{2\pi}{8\pi}=\dfrac{1}{4}(s)\\\longrightarrow A.\)

Câu 7:

\(x=20\cos\left(2\pi t+\dfrac{\pi}{4}\right)\\t=0,25(s)\Rightarrow x=20\cos\left(2\pi .0,25+\dfrac{\pi}{4}\right)=10\sqrt{2}\approx 14,14 (mm)\\\longrightarrow D.\)

Câu 8:

\(x=5\sin\left(5\pi t+\dfrac{\pi}{4}\right)=5\cos\left(5\pi t+\dfrac{\pi}{4}-\dfrac{\pi}{2}\right)=5\cos\left(5\pi t-\dfrac{\pi}{4}\right)\\\Rightarrow \begin{cases} A=5(cm)\\\omega =5\pi (rad/s)\\T=\dfrac{2\pi}{\omega}=\dfrac{2\pi}{5\pi}=\dfrac{2}{5}=0,4(s)\\f=\dfrac{1}{T}=\dfrac{5}{2}=2,5(hz)\end{cases}\\\longrightarrow C.\)