An alternate definition of convex quadrilateral. The opposite sides AB and CD of a quadrilateral nABCD are said to be semiparallel if AB _ ('CD D _ and CD _ ('AB D _. Prove the following two results. (a) If one pair of opposite sides of nABCD is semiparallel, then the other is. (b) nABCD is convex if and only if both pairs of opposite sides are semiparallel.