Solution
= Solution
{c}
The <cross product> $\mathbf b\times\mathbf c$ is perpendicular to both factors. Thus
$$
\boxed{\mathbf a'\cdot\mathbf b=\frac{(\mathbf b\times\mathbf c)\cdot\mathbf b}{V}=0,\qquad\mathbf a'\cdot\mathbf c=\frac{(\mathbf b\times\mathbf c)\cdot\mathbf c}{V}=0.}
$$
Cyclic permutations give all the off-diagonal <dot products> between the original <basis> and its <reciprocal basis>.