Question

surveying crew located three points on a map that formed the vertices of a triangular area. A coordinate grid in which on unit equals 20 miles is place over the map so that the vertices are located at (0, -1), (-3, -9), and (3, -2). Find the area of the triangle using determinants?

Answer 1

5400 mi²

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 = ...

`\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`

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