_Sad boy_
 

Đáp án+Giải thích các bước giải:
 Có: $$x=5cos\left(20t\right) + 5sin\left(20t \right)$$
$$⇔x=5\left[cos\left(20t\right) + sin\left(20t \right)\right]$$
$$⇔5\sqrt{2}cos\left(20t-\dfrac{\pi}{4}\right)$$
$$⇒A=5\sqrt{2}(cm), T=\dfrac{2\pi}{\omega}=\dfrac{2\pi}{20}=\dfrac{\pi}{10}(s), f=\dfrac{1}{T}=\dfrac{1}{\dfrac{\pi}{10}}=\dfrac{10}{\pi}(\text{Hz})$$