For instance if O is the point , A is and B is then the edges OA and OB are represented by the vectors and respectively. The area of the face OAB is then , and this vector is perpendicular to the plane OAB, in a direction given by the right-hand rule. But you need this vector to be pointing outwards from the tetrahedron, and, if you sketch a diagram, you'll see that it points inwards (in the positive direction of ). So, in fact, the vector you need is , which is given by
If you call C the point , and , then you can find the vectors for the areas of the faces OAC and OBC in the same way.
To find the area of the face ABC, you'll first need vectors representing two of its edges. For instance , and .
For more on how this works see Cross product - Wikipedia, the free encyclopedia
Can you complete it OK now?