1.28. Given two points $A$ and $B$ and a positive number $k \neq 1$. Find the geometric locus of points $M$ (GML) such that $A M: B M=k$.
1.28. Introduce a coordinate system such that points $A$ and $B$ have coordinates $( -a, 0,0 )$ and $( a, 0,0)$ respectively. If point $M$ has coordinates $(x, y, z)$, then $\frac{A M^{2}}{B M^{2}}=\frac{(x+n)^{2}+y^{2}+z^{2}}{(x-a)^{2}+y^{2}+z^{2}}$. The equation $A M: B M=k$ can be transformed into the form

$$
\left(x+\frac{1+k^{2}}{1-k^{2}} a\right)^{2}+y^{2}+z^{2}=\left(\frac{2 k a}{1-k^{2}}\right)^{2}
$$

This equation represents a sphere with center $\left(-\frac{1+k^{2}{ }^{2}}{1-k^{2}} a, 0,0\right)$ and radius $\left|\frac{2 k a}{1-k^{2}}\right|$.