With α = 30° and the formula
`(sin alpha)/2=+-sqrt((1-cos a)/2`
`sin 15^text(o)=+-sqrt((1-cos 30^text(o))/2)` `=+-sqrt((1-0.866...)/2)` `=0.2588`
Here the positive value is chosen because 15° is in the first quadrant.
[Why bother doing this when we can use a calculator? This is just to illustrate that the formula works.]