Solution 4.4:1a

From Förberedande kurs i matematik 1

In the unit circle's first quadrant, there is one angle whose sine value equals 21 and that is v=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 v=6 makes with the positive x -axis, i.e. the other angle is v=−6=56