theorem :: NIVEN:56

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