##sin^(-1)(sqrt(3)/2) = pi/3## (radians)

##sin^(-1)(sqrt(3)/2) = theta##

##color(white)(XX)##means

##sin(theta)=sqrt(3)/2##

##color(white)(XXXX)##with the restriction that ##theta in [-pi/2pi/2]##

The possible triangles with this ratio of ##y## to ##hypotenuse## (i.e. ##sin##) are pictured below:

…but note that only ##pi/3## falls within the restricted range for ##sin^(-1)##