Determine the area of the quadrilateral whose vertices are (1,3), (6,5), (9,1) and
(1) sum of all the [ a numerator multiplied by the next denominator]
(2) sum of all the [ denominator multiplied by the next numerator ]
subtract the second sum from the first (-7 -63 = -70)
and divide that absolute value by 2 ( 70/2 = 35 )
This method will calculate the area of any polygon, when you know the coordinates of the points around the polygon. If you proceed in a clockwise direction around the perimeter (as was done above) you will have negative quantity; If you proceed in a counterclockwise direction the resulting quantity will be positive.