Question 1101104
Find the area of the quadrilateral having points A(-3,4), B(-2,-1), C(4,4) and D(0,5) as vertices
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<pre>
 A   B   C   D   A
-3  -2   4   0  -3
 4  -1   4   5   4
</pre>
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Add the diagonal products starting at the upper left.
--> 3 -8 +20 +0 = 15
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Add the diagonal products starting at the lower left.
--> -8 -4 +0 -15 = -27
The difference is 42
The area is 1/2 that, = 21 sq units
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This method works for all quad's, but the points have to be entered in order around the quad.
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All polygons, not just for 4 sides.  Any number of points.