Consider all segments dividing the area of a triangle $ABC$ in two equal parts. Find the length of the shortest segment among them, if the side lengths $a,$ $b,$ $c$ of triangle $ABC$ are given. How many of these shortest segments exist ?
1. **Identify the problem and given data:**
   We are given a triangle \(ABC\) with side lengths \(a\), \(b\), and \(c\). We need to find the shortest segment that divides the area of the triangle into two equal parts and determine how many such shortest segments exist.

2. **Set up the problem:**
   Consider a point \(D\) on one of the sides of the triangle, say \(BC\). Let \(P\) and \(Q\) be points on the sides \(AB\) and \(AC\) respectively, such that the segment \(PQ\) divides the triangle into two equal areas.

3. **Area division:**
   The area of triangle \(ABC\) is \(S\). The segment \(PQ\) divides the triangle into two regions, each with area \(\frac{S}{2}\).

4. **Using trigonometry:**
   Let \(DP = x\) and \(DQ = y\). The area of triangle \(DPQ\) is given by:
   \[
   \frac{1}{2} \cdot x \cdot y \cdot \sin(\angle PDQ) = \frac{S}{2}
   \]
   Simplifying, we get:
   \[
   xy \sin(\angle PDQ) = S
   \]
   Since \(\sin(\angle PDQ)\) is a constant for a given triangle, we can denote it as \(k\). Thus:
   \[
   xy = \frac{S}{k}
   \]

5. **Minimizing the length of \(PQ\):**
   The length of \(PQ\) can be found using the Law of Cosines:
   \[
   PQ^2 = x^2 + y^2 - 2xy \cos(\angle PDQ)
   \]
   To minimize \(PQ\), we need to minimize \(x^2 + y^2\) given that \(xy\) is a constant. 

6. **Using the AM-GM inequality:**
   By the Arithmetic Mean-Geometric Mean Inequality (AM-GM Inequality):
   \[
   \frac{x^2 + y^2}{2} \geq \sqrt{x^2 y^2}
   \]
   Simplifying, we get:
   \[
   x^2 + y^2 \geq 2xy
   \]
   Since \(xy = \frac{S}{k}\), we have:
   \[
   x^2 + y^2 \geq 2 \cdot \frac{S}{k}
   \]
   The equality holds when \(x = y\). Therefore, the minimum value of \(x^2 + y^2\) is achieved when \(x = y\).

7. **Finding the minimum length:**
   When \(x = y\), we have:
   \[
   x = y = \sqrt{\frac{S}{k}}
   \]
   Substituting \(x = y\) into the expression for \(PQ\):
   \[
   PQ = \sqrt{x^2 + y^2 - 2xy \cos(\angle PDQ)} = \sqrt{2x^2 - 2x^2 \cos(\angle PDQ)} = x \sqrt{2(1 - \cos(\angle PDQ))}
   \]
   Using the identity \(1 - \cos(\angle PDQ) = 2 \sin^2\left(\frac{\angle PDQ}{2}\right)\), we get:
   \[
   PQ = x \sqrt{4 \sin^2\left(\frac{\angle PDQ}{2}\right)} = 2x \sin\left(\frac{\angle PDQ}{2}\right)
   \]
   Substituting \(x = \sqrt{\frac{S}{k}}\):
   \[
   PQ = 2 \sqrt{\frac{S}{k}} \sin\left(\frac{\angle PDQ}{2}\right)
   \]

8. **Conclusion:**
   The shortest segment \(PQ\) is achieved when \(x = y\), and its length is:
   \[
   PQ = 2 \sqrt{\frac{S}{k}} \sin\left(\frac{\angle PDQ}{2}\right)
   \]
   Since there are three vertices in the triangle, there are three such shortest segments, one for each vertex.

The final answer is \( \boxed{ PQ = 2 \sqrt{\frac{S}{k}} \sin\left(\frac{\angle PDQ}{2}\right) } \) and there are 3 such shortest segments.