theorem :: NIVEN:60

for r being Real
for i being Integer st ((3 * PI) / 2) + ((2 * PI) * i) <= r & r <= (2 * PI) + ((2 * PI) * i) & r / PI is rational & cos r is rational holds
r in {(((3 * PI) / 2) + ((2 * PI) * i)),(((5 * PI) / 3) + ((2 * PI) * i)),((2 * PI) + ((2 * PI) * i))}