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I'm trying to figure out what the formula is to find the surface area of a non-regular tetrahedron when given the vertices coordinates.

Example:

Vertex A: [0, 0, 5]

Vertex B: [-1, -1, 0]

Vertex C: [1, 0, 0]

Vertex D: [0, 2, 0]

Basically I want to turn this user input into surface area.

Thank you

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2 Answers 2

Hint: As a numerical analyst, the most common way to compute the face are of a bunch of tetrahedra is using cross product, since it is the most easily vectorized/paralleled algorithm. The face opposite to $A$ denoted as $F_A$ is spanned by the vector $\overrightarrow{BC}$ and $\overrightarrow{BD}$, then the area is $$ |F_A| = \frac{1}{2} |\overrightarrow{BC}\times \overrightarrow{BD}| $$

More likely since you mentioned "input", I am guessing you are writing some subroutine, if using cyclic notation that a tetrahedron has vertices $V_i$, $i=1,\cdots,4$, then above formula can be vectorized using the following MATLAB code snippets assuming you have your $i$-th vertex of the $n$-th tetrahedron stored in a 3d-array V(n,:,i):

face_normal = cross(V(:,:,i+1) - V(:,:,i-1), ...
V(:,:,i+1) - V(:,:,i+2), 2);
face_area = 0.5*sqrt(sum(face_normal.^2,2));

which could be easily ported to Python, C or Fortran.

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Yep, this is just a part of a larger C program. That's a little more advanced than I'm looking for. This is just for ece class trying to teach us how to program in C. –  apichel Apr 26 '12 at 21:18
    
@apichel Good to know your exact need! Then I suggest first build a subroutine for the cross product, if you are not allowed to use any other library than stdio and stdlib. Moreover if you could set an array storing the information of the faces, like F[n][j] is the index in the original vertex array for the $j$-th vertex in the $n$-th face, that would further ease your computation. –  Shuhao Cao Apr 26 '12 at 22:08

You have four triangles for which you can compute the sides from the Pythagorean theorem. For example, $CD=\sqrt{1^2+2^2+0^2}=\sqrt 5$. Then use Heron's formula

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So I would need to find AB, AC, AD, BC, BD, and CD. Then plug into Heron's, where a = AB, b = BD, and c = AD? –  apichel Apr 26 '12 at 21:00
    
Exactly. That gets you face ABD. Each set of three edges chosen from the four represents one face of the tetrahedron. –  Ross Millikan Apr 26 '12 at 21:03

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