<PRE><B>Question 587246</B>
What is the area of the quadrilateral with vertices A(0,2), B(15,10), C (15,2), &amp; D(0,-18)?
-------------
Make a matrix, 2 by P, the number of points, repeat the first point.
A...B...C...D...A
0..15..15...0...0
2..10...2..-18..2
---------------------
Add the diagonal products starting at the upper left:
0 + 30 - 270 + 0 = -240
Add the diagonal products starting at the lower left:
30 + 150 + 0 + 0 = 180
--------
The area is 1/2 the ABS of the difference
|-240 - 180| = 420
Area = 210 sq units.
----------------------
It works for all polygons, any number of sides.
</PRE>