Trivial Proof of the formula used to find triangle in coordinate geometry (which can be extended to n-sided polygons)

Let say we want to find the area of the pink triangle (A+B) given the coordinates of the three vertices

Similarly, we can prove by the box method and construct a different diagram to obtain the area a:

Cheers, 

Euclid

P.S sorry for the messy print screens. The equations cant show up properly in the text box.