Here's a method of finding the area of any polygon knowing its vertices. I'm using this parallelogram as an example.
Make a table like this (each vertex with its x- and y-coordinates):
Pt x y
A 3 6
B 6 5
C 5 1
D 2 2
A 3 6
Now multiply each x-coordinate by the y-coordinate on the line below and write it on the right side. Bold type shows the first multiplication.
x y
A 3 6
B 6 515
C 5 1 6
D 2 2 10
A 3 6 12
Now multiply each y-coordinate by the x-coordinate on the line below and subtract from each produce you already have. Do each subtraction. Bold type shows the first multiplication.
x y
A 3 6
B 6 5 15 - 36 = -21
C 5 1 6 - 25 = -19
D 2 2 10 - 2 = 8
A 3 6 12 - 6 = 6
Add all the differences.
x y
A 3 6
B 6 5 15 - 36 = -21
C 5 1 6 - 25 = -19
D 2 2 10 - 2 = 8
A 3 6 12 - 6 = 6
+____
-26
The area of the polygon is the absolute value of half of the sum of the differences.
