In a unit circle, the y co-ordinate is .
If this is positive, then the y co-ordinate is above the x-axis.
If you draw a horizontal line through this y-axis value,
it cuts the circle in two places.
Calculating inverse sine gives you the acute angle the point on the circle makes with the positive x-axis.
As the situation is symmetrical, how do you calculate the answer that
your calculator does not give you?