Areas and volumes of parallelograms and parallelepipeds in 3 dimensions are often easily found by making use of the cross product of the direction vectors of their edges. For edge vectors v1 and v2 of a triangle, the area is ...
... A = (1/2)║v1 × v2║
that is, half the norm of the cross-product vector. The area of a parallelogram with those edge vectors is simply ...
... A = ║v1 × v2║
Here, direction vectors are ...
- ab = (-5, 2, 0)
- bc = (-5, 1, 5)
- cd = (5, -2, 0)
- da = (5, -1, -5)
We can see that ab = -cd and bc = -da, as required for a parallelogram.
The cross product ab × bc is (10, 25, 5), so the area of the parallelogram is
... ║(10, 25, 5)║ = √(10² +25² +5²) = √750
... Area = 5√30 ≈ 27.3861 . . . . square units (parallelogram area)
The areas of each of the mentioned triangles is half the area of the parallelogram, so is
... Area Δabc = Area ∆abd = (5/2)√30 ≈ 13.6931 . . . . square units (triangles)
