<PRE><B>Question 1197252</B>
find the are of a quadrilateral having points A(5,2), B(4,3), C(2,4), D(-8,-1) as consecutive vertices.
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&lt;pre&gt;
 A   B   C   D   A
 5   4   2  -8   5
 2   3   4  -1   2
&lt;/pre&gt;
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Add the diagonal products starting at the upper left:
5*3 + 4*4 + 2*-1 + -8*2 = 15 + 16 - 2 - 16 = 13
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Add the diagonal products starting at the lower left:
2*4 + 3*2 + 4*-8 + -1*5 = 8 + 6 -32 - 5 = -23
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The difference between the sums is 36.
1/2 of 36 is the area
Area = 18 sq units

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