Mr. and Mrs. Richard Lee Alexander of Warm Springs, Georgia, announce the engagement of their daughter, Sage Elizabeth Alexander, to H. Smith Mathews Jr., both of Savannah, Georgia. Mathews is the son of Mr. and Mrs. Harry Smith Mathews of Statesboro. The bride-elect is the granddaughter of Dr. Paul Alexander and the late Dora Alexander of Waynesville, North Carolina, and Dr. and Mrs. James Campbell of Cantonsville, Maryland.