For that you must restrict the angle only from 0 to 180 degrees. What this means is whatever answer you get must be within this range. For eg. (1, 150) will have another 3 answers for the same if theta is from 0 to 360. But is it is restricted to 0 to 180, then only (1, 150) is correct answer because the angle must be within 0 to 180 degrees.