Elementary Geometry for College Students (7th Edition) Clone

60$^{\circ}$
$\angle$STR=90 because it is inscribed in a semicircle. Since ST is equal to half SR (the hypotenuse of STR), then STR is a 30 60 90 triangle. Arc ST=2$\angle$SRT Arc ST=2(30)=60$^{\circ}$