In a triangle, a straight line intersects two of its sides in distinct points and is parallel to the third side.
(i) State the theorem that relates the ratios of the two pairs of segments formed on those sides.
(ii) In △PQR, points S and T lie on PQ and PR respectively such that ST∥QR. If PS:SQ=2:3, show that PT:TR=2:3.
(iii) Give a short reason why the ratios obtained in part (ii) are equal.