Solution
= Solution
For $r=0$, the <norm> equation requires $x=c$. Thus the <locus> is
$$
\boxed{\{c\}\text{ if }a\cdot(c-b)=0;\qquad\varnothing\text{ otherwise}.}
$$
This includes $a=0$, when the first condition imposes no restriction.
= Solution
For $r=0$, the <norm> equation requires $x=c$. Thus the <locus> is
$$
\boxed{\{c\}\text{ if }a\cdot(c-b)=0;\qquad\varnothing\text{ otherwise}.}
$$
This includes $a=0$, when the first condition imposes no restriction.