From Förberedande kurs i matematik 1

In the unit circle's first quadrant, there is one angle whose sine value equals 1/2 and that is \displaystyle v = \pi/6\,.

From the figures, we see that there is a further angle with the same sine value and it lies in the second quadrant. Because of symmetry, it makes the same angle with the negative x-axis as \displaystyle v=\pi/6 makes with the positive x-axis, i.e. the other angle is \displaystyle v = \pi - \pi/6 = 5\pi/6\,.