$(MON 5)$ Find the radius of the circle circumscribed about the isosceles triangle whose sides are the solutions of the equation $x^2 - ax + b = 0$.
To find the radius of the circle circumscribed about the isosceles triangle whose sides are the solutions of the equation \(x^2 - ax + b = 0\), we need to follow these steps:

1. **Identify the roots of the quadratic equation:**
   The roots of the quadratic equation \(x^2 - ax + b = 0\) are given by:
   \[
   x = \frac{a \pm \sqrt{a^2 - 4b}}{2}
   \]
   Let's denote the roots as \(r_1\) and \(r_2\). Therefore, \(r_1 = \frac{a + \sqrt{a^2 - 4b}}{2}\) and \(r_2 = \frac{a - \sqrt{a^2 - 4b}}{2}\).

2. **Determine the sides of the isosceles triangle:**
   Since the triangle is isosceles, two of its sides are equal. Let the equal sides be \(r_1\) and \(r_2\), and the base be \(r_3\). The third side \(r_3\) must also be a root of the quadratic equation, so \(r_3 = r_1\) or \(r_3 = r_2\).

3. **Use the circumradius formula for a triangle:**
   The formula for the circumradius \(R\) of a triangle with sides \(a\), \(b\), and \(c\) is:
   \[
   R = \frac{abc}{4K}
   \]
   where \(K\) is the area of the triangle.

4. **Calculate the area \(K\) of the isosceles triangle:**
   For an isosceles triangle with sides \(a\), \(a\), and \(b\), the area \(K\) can be calculated using the formula:
   \[
   K = \frac{b}{4} \sqrt{4a^2 - b^2}
   \]
   Here, \(a = r_1\) and \(b = r_2\).

5. **Substitute the values into the circumradius formula:**
   Since \(a = r_1\) and \(b = r_2\), we have:
   \[
   R = \frac{r_1 \cdot r_1 \cdot r_2}{4 \cdot \frac{r_2}{4} \sqrt{4r_1^2 - r_2^2}}
   \]
   Simplifying this expression, we get:
   \[
   R = \frac{r_1^2 \cdot r_2}{r_2 \sqrt{4r_1^2 - r_2^2}}
   \]
   \[
   R = \frac{r_1^2}{\sqrt{4r_1^2 - r_2^2}}
   \]

6. **Simplify the expression:**
   Since \(r_1\) and \(r_2\) are the roots of the quadratic equation, we know that \(r_1 + r_2 = a\) and \(r_1 \cdot r_2 = b\). Therefore, we can rewrite the expression for \(R\) as:
   \[
   R = \frac{\left(\frac{a + \sqrt{a^2 - 4b}}{2}\right)^2}{\sqrt{4\left(\frac{a + \sqrt{a^2 - 4b}}{2}\right)^2 - \left(\frac{a - \sqrt{a^2 - 4b}}{2}\right)^2}}
   \]

7. **Further simplification:**
   Simplifying the expression inside the square root, we get:
   \[
   R = \frac{\left(\frac{a + \sqrt{a^2 - 4b}}{2}\right)^2}{\sqrt{4\left(\frac{a + \sqrt{a^2 - 4b}}{2}\right)^2 - \left(\frac{a - \sqrt{a^2 - 4b}}{2}\right)^2}}
   \]
   \[
   R = \frac{\left(\frac{a + \sqrt{a^2 - 4b}}{2}\right)^2}{\sqrt{4\left(\frac{a^2 + 2a\sqrt{a^2 - 4b} + (a^2 - 4b)}{4}\right) - \left(\frac{a^2 - 2a\sqrt{a^2 - 4b} + (a^2 - 4b)}{4}\right)}}
   \]
   \[
   R = \frac{\left(\frac{a + \sqrt{a^2 - 4b}}{2}\right)^2}{\sqrt{4\left(\frac{2a\sqrt{a^2 - 4b}}{4}\right)}}
   \]
   \[
   R = \frac{\left(\frac{a + \sqrt{a^2 - 4b}}{2}\right)^2}{\sqrt{a\sqrt{a^2 - 4b}}}
   \]
   \[
   R = \frac{\left(\frac{a + \sqrt{a^2 - 4b}}{2}\right)^2}{\sqrt{a\sqrt{a^2 - 4b}}}
   \]

8. **Final expression for the circumradius:**
   After simplifying, we get:
   \[
   R = \frac{(a + \sqrt{a^2 - 4b})^2}{4a\sqrt{a^2 - 4b}}
   \]

The final answer is \(\boxed{\frac{(a + \sqrt{a^2 - 4b})^2}{4a\sqrt{a^2 - 4b}}}\)