Solution
= Solution
The orientation assumption gives $\mathbf c\cdot(\mathbf a\times\mathbf b)>0$. The same open <half-space> is therefore characterized by
$$
\boxed{\mathbf r\cdot(\mathbf a\times\mathbf b)>0}.
$$
Equality would put the point in the separating <plane>, rather than strictly on the same side.