\(x=10cos\left(10\pi t-\dfrac{\pi}{4}\right)\left(cm\right)\left(1\right)\)
\(a=x''=-1000\pi^2cos\left(10\pi t-\dfrac{\pi}{4}\right)\left(2\right)\)
\(x=-A=-10\left(cm\right)\)
\(\left(1\right)\Rightarrow cos\left(10\pi-\dfrac{\pi}{4}\right)=-1\)
\(\left(2\right)\Rightarrow a=-1000\pi^2.\left(-1\right)\sim10000\left(cm/s^2\right)\)