Solution
= Solution
The magnitude of the <cross product> of two edge vectors is the area of their <parallelogram>. The triangular face occupies half of it:
$$
\boxed{\operatorname{area}(PQR)=\frac12|(-1,1,1)\times(-2,-1,2)|=\frac{3\sqrt2}{2}.}
$$
= Solution
The magnitude of the <cross product> of two edge vectors is the area of their <parallelogram>. The triangular face occupies half of it:
$$
\boxed{\operatorname{area}(PQR)=\frac12|(-1,1,1)\times(-2,-1,2)|=\frac{3\sqrt2}{2}.}
$$