How do I find the value of p (t)= (-200sin (20π t))×(10cos (20πt))
$$\\=-2000sin (20\pi t)cos (20\pi t))\\\\=-1000*2sin (20\pi t)cos (20\pi t))\\\\=-1000\;sin (40\pi t)\\\\$$
It t is an integer than this equals 0
otherwise I think it is fully simplified.