The area can be found as the magnitude of the cross product of the vectors representing two adjacent sides.
area = || kl × kn || = || (0, 2, 1) × (4, 5, 0) ||
= || (-5, 4, -8) || =
√105_____
For the purpose here, kl = l - k = (1, 3, 2) - (1, 1, 1) = (0, 2, 1)
and nk = n - k = (5, 6, 1) - (1, 1, 1) = (4, 5, 0)
The cross product can be calculated using a suitable calculator. By hand, it is the determinant
![\left|\begin{array}{ccc}i&j&k\\0&2&1\\4&5&0\end{array}\right|](https://tex.z-dn.net/?f=%5Cleft%7C%5Cbegin%7Barray%7D%7Bccc%7Di%26j%26k%5C%5C0%262%261%5C%5C4%265%260%5Cend%7Barray%7D%5Cright%7C)
where i, j, k are unit vectors in the x, y, z directions.