Area of the quadrilateral can be found by splitting a quadrilateral into two triangles by drawing one of its diagonals.
Area of a quadrilateral can be found by splitting it into triangles.
If a quadrilateral is split into two triangles by drawing a diagonal then the area of the quadrilateral is equal to the sum of the areas of the triangles.
I draw a quadrilateral and then draw all its diagonals. How many triangles can I find?