Solution
= Solution
A <norm> is nonnegative, so it cannot equal a negative $r$. The <locus> is therefore $\boxed{\varnothing}$, irrespective of $a,b,c$.
= Solution
A <norm> is nonnegative, so it cannot equal a negative $r$. The <locus> is therefore $\boxed{\varnothing}$, irrespective of $a,b,c$.