In △PQR, points S and T are chosen on PQ and PR respectively so that PS:SQ=PT:TR.
a. (i) Explain with reasons whether the information given is sufficient to conclude that ST∥QR.
b. (ii) If your answer to part (i) is negative, specify the additional verification that must be carried out before arriving at a conclusion about the parallelism of ST and QR.
c. (iii) Name the geometrical principle that finally justifies the conclusion you reach.