Question 1185345
Find the area of a quadrilateral having the 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
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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
</pre>
The difference is 42.
The area is 1/2 that = 21 sq units.