What does "union" mean in a geometry class?

In Geometry "Union" is normally used to indicate the combining of several geometric shapes into a single shape.

Consider the example below. The concept of geometric unions is used (incompletely here) to demonstrate why the area of a triangle is ##1/(wh)##

The following geometric union statements are applicable:

##color(white)("XXXX")####squareACDF =triangleAB_1Fuutriangle FBDuutriangleDB_2C##

##color(white)"XXXX"####triangle BFD = triangle FB_1B_2 = triangle FB_1AuutriangleDB_2C##