An angle whose vertex is on a circle and whose sides are chords of the circle is an inscribed angle.
Therefore, in the given diagram, ∠ S is an inscribed angle. The intercepted arc of this angle has a measure of RT.
Note that the endpoints of RTS form a diameter of the circle, therefore, it measures 180^(∘). We can use the Arc Addition Postulate to find the value of RT.
According to the Inscribed Angle Theorem, the measure of an inscribed angle is half the measure of its intercepted arc.
This means that m∠ S is half of mRT.
