Area of a triangle
Here are seven (slightly) different ways to calculate the area of a triangle:
1. Area = a*h/2
2. Area = a*b*sin(C)/2

3. Area = a2*sin(B)*sin(C)/(2*sin(B+C))

4. Area = sqrt(s*(s-a)*(s-b)*(s-c))
where s=(a+b+c)/2 is the semi-perimeter.

5. Area = f*g/2 -v*w/2
6. Area = abs((xB*yA-xA*yB)+(xC*yB-xB*yC)+(xA*yC-xC*yA))/2

If the vertices are at integer points on a grid of points then there is another:
7. Area = number of points inside triangle + half number of points on edge of triangle -1
http://www.btinternet.com/~se16/hgb/triangle.htm