\(x=0\)
\(\Leftrightarrow5cos\left(\pi t+\dfrac{\pi}{2}\right)=0\)
\(\Leftrightarrow cos\left(\pi t+\dfrac{\pi}{2}\right)=0\)
\(\Leftrightarrow\pi t+\dfrac{\pi}{2}=\dfrac{\pi}{2}+k\pi\)
\(\Leftrightarrow t=k\)
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