Given an isosceles triangle. Find the set of the points inside the triangle such, that the distance from that point to the base equals to the geometric mean of the distances to the sides.
1. Consider an isosceles triangle \(ABC\) with \(AB = AC\). Let \(P\) be a point inside the triangle \(ABC\).
2. Let the feet of the perpendiculars dropped from \(P\) to the sides \(AB\), \(BC\), and \(CA\) be \(M\), \(K\), and \(L\), respectively.
3. We are given that \(PK = \sqrt{PM \cdot PL}\). This implies that \(\frac{PM}{PK} = \frac{PK}{PL}\).
4. Quadrilaterals \(PMBK\) and \(PKCL\) are cyclic. This implies that \(\angle MPK = 180^\circ - \angle MBK\) and \(\angle KPL = 180^\circ - \angle KCL\).
5. Since \(\angle MPK = \angle KPL\) and \(\frac{PM}{PK} = \frac{PK}{PL}\), by the Side-Angle-Side (SAS) similarity criterion, we get \(\triangle MPK \sim \triangle KPL\).
6. From the similarity, we have \(\angle MBP = \angle MKP = \angle KLP = \angle PCB\) and \(\angle LCP = \angle LKP = \angle PMK = \angle PBC\).
7. This implies that the circumscribed circle of the triangle \(BPC\) touches the sides \(AB\) and \(AC\) at the points \(B\) and \(C\), respectively.
8. Therefore, the locus of the points \(P\) is the minor arc \(BC\) of the circle tangent to the sides \(AB\) and \(AC\) at the points \(B\) and \(C\), respectively.

The final answer is the minor arc \( \boxed{ BC } \) of the circle tangent to the sides \(AB\) and \(AC\) at the points \(B\) and \(C\), respectively.