不等邊四邊形面積計法？

Try Bramhaguta's formula. It will give you the area of a quadrilateral as long as it is not a concave type.

area = sqrt{(s-a)(s-b)(s-c)(s-d)}

where s, the semiperimeter, is
s=(a+b+c+d)/2

a, b, c, d are the lengths of four sides of the quadrilateral. For more information:

http://en.wikipedia.org/wiki/Brahmagupta%27s_formula