Show that the following is a convex set (vectors)?

A set of vectors is called a convex set if the following holds: Whenver U and V belong to the set, so does AU + (1-A)V for any scalar A between 0 and 1. Consider set Ra,b with one corner at origin and far corner at point <a,b>. Ra,b = {<x,y>|o<x<a, 0<y<b} (the < denotes less than or equal to) Show that Ra,b is a convex set.