a math

Area
=(1/2)[(3*2+0*0+-1*0+4*3)-(3*0+2*-1+0*4+0*3]
=10//

In anti-clockwise direction, the vertices are (0,2),(-1,0),(4,0),(3,3),(0,2)
Area of the quadrilateral
= 1/2 [0*0+(-1)*0+4*3+3*2-(-1)*2-4*0-3*0-0*3]
=1/2 (20)
=10

Solution:

First, you should join (-1,0) and (0,2) . Next, join (0,2) and (3,3) . Last, join (3,3) and (4,0). After you have joined these lines, you will be able to see two triangles and a trapezium. Calculate each of the polygon and add them to together will done this calculation.


圖片參考：http://www.freewebtown.com/dennistang/knowledge/images/math.png

From the above, 1 represents area of the triangle on the second quadrant, the rest are the polygons on the first quadrant.

The area is the sum of those three numbers shown, i.e.

1.5 7.5 1

Since that, the area is equal to 1.5 7.5 1.
i.e. Area = 1.5 7.5 1
= 10 cm2