= Solution
If $x\in\operatorname{Null}(A)$, the $i$th entry of $Ax=0$ says $r_i\cdot x=0$. Any $y\in\operatorname{Row}(A)$ is a <linear combination> $y=\sum_i\alpha_i r_i$, so bilinearity of the <dot product> gives
$$
y\cdot x=\sum_i\alpha_i(r_i\cdot x)=0.
$$
Hence
$$
\boxed{\operatorname{Row}(A)\perp\operatorname{Null}(A).}
$$
This proves the <orthogonality> for every pair of <vectors> in the two <vector subspaces>. In fact the <null space> equals the <orthogonal complement> of the <row space>, since being <orthogonal> to every row is exactly the system $Ax=0$.
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