In a circle with centre O, chord PQ subtends an angle of 150∘ at the centre. A point R lies on the major arc PQ and a point S lies on the minor arc PQ.
a. (i) Find the measure of ∠PRQ.
b. (ii) Find the measure of ∠PSQ.
c. (iii) Prove that ∠PRQ+∠PSQ=180∘ for this configuration.