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http://en.wikipedia.org/wiki/Polygon

Here is a solution using the above that could be used for a simple polygon
with an arbitrary # of points:

       def area
               p0  c
               area2  
               [@a, @b, @c].each { |p|
                   area2 + 0[0]*p[1]-p[0]*p0[1]
                   p0  
               }
               (area2/2.0).abs
       end

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