In some cases, and this may be one of them, the best answer may be simply to memorize it.
You should be acquainted with some basic values on the Unit Circle. The ones you REALLY should know are sine and cosine for 0, , , , and . If you have all these stuck in your head, you can use various symmetries and equations to derive their relatives in the first quadrant and the other three quadrants.
Since we just memorized , what does that make asin(1/2)?