Also, this should be written to take advantage of numpy slicing:

```python
In [22]: %timeit abs(np.linalg.det([a[0]-a[1], a[1]-a[2], a[2]-a[3]])) / 6.0
10000 loops, best of 3: 25.1 µs per loop

In [23]: %timeit np.abs(np.linalg.det(a[:-1] - a[1:])) / 6.0
10000 loops, best of 3: 19.6 µs per loop
```