The Area of Triangle

1. Coordinates


△ABC
= ADEC - △ADB - △BEC
= 1/2(x1y2 + x2y3 + x3y1 - x2y1 - x3y2 - x1y3)
= 

2. Heron's Formula
△＝ sqrt(s(s-a)(s-b)(s-c))
x = 1/2 (a+b+c)



Ref.:
http://en.wikipedia.org/wiki/Triangle
http://euler.tn.edu.tw/area.pdf