Answer:
5400 mi²
Step-by-step explanation:
The area is half the absolute value of the sum of the determinants of the points taken pairwise in some consistent order around the polygon. For a triangle, this sum of determinants can also be written as the determinant of a 3×3 matrix of the coordinates with the third column being 1.
... Area = (1/2)|D|
where D = ...
[tex]\left|\begin{array}{ccc}0&-1&1\\-3&-9&1\\3&-2&1\end{array}\right|=\left|\begin{array}{cc}0&-1\\-3&-9\end{array}\right|+\left|\begin{array}{cc}-3&-9\\3&-2\end{array}\right|+\left|\begin{array}{cc}3&-2\\0&-1\end{array}\right|\\\\=-3+33-3=27[/tex]
Then the area is (in grid squares) ...
... Area = (1/2)|27| = 13.5 . . . . grid squares
Each grid square is (20 mi)² = 400 mi², so 13.5 grid squares is ...
... 13.5 × 400 mi² = 5400 mi²
