use geometry.frink

// Return a polygon object representing the specified equilateral triangle.
// THINK ABOUT:  Would this be better as a GeneralPath of 4 disconnected
// polygons?
makeEquilateralTriangle[cx, cy, sideLength, rotation] :=
{
   p = new polygon
   halfL = 1/2 sideLength
   h = halfL sqrt[3]
   halfH = 1/2 h

   x1 = cx
   y1 = cy - halfH
   [x2, y2] = rotateAroundPoint[x1,y1,cx,cy,rotation]
   p.addPoint[x2, y2]

   x1 = cx - halfL
   y1 = cy + halfH
   [x2, y2] = rotateAroundPoint[x1,y1,cx,cy,rotation]
   p.addPoint[x2, y2]

   x1 = cx + halfL
   [x2, y2] = rotateAroundPoint[x1,y1,cx,cy,rotation]
   p.addPoint[x2, y2]

   return p
}

// Draw the specified tetrahedron into the graphics object.
makeTetrahedron[g is graphics, cx, cy, sideLength] :=
{
   hl = sideLength / 2
   ho = hl sqrt[3]
   g.add[makeEquilateralTriangle[cx,           cy, sideLength, 0 deg]]
   g.add[makeEquilateralTriangle[cx,      cy + ho, sideLength, 180 deg]]
   g.add[makeEquilateralTriangle[cx + hl, cy + ho, sideLength, 0 deg]]
   g.add[makeEquilateralTriangle[cx - hl, cy + ho, sideLength, 0 deg]]
}

g = new graphics
makeTetrahedron[g, 0, 0, 1]
g.show[]