3 points $ O(0,\ 0),\ P(a,\ a^2), Q( \minus{} b,\ b^2)\ (a > 0,\ b > 0)$ are on the parabpla $ y \equal{} x^2$.
Let $ S_1$ be the area bounded by the line $ PQ$ and the parabola and let $ S_2$ be the area of the triangle $ OPQ$.

Find the minimum value of $ \frac {S_1}{S_2}$.
1. **Equation of the Line \(PQ\):**
   The points \(P(a, a^2)\) and \(Q(-b, b^2)\) lie on the parabola \(y = x^2\). The equation of the line passing through these points can be found using the slope-intercept form. The slope \(m\) of the line \(PQ\) is given by:
   \[
   m = \frac{b^2 - a^2}{-b - a} = \frac{(b - a)(b + a)}{-(b + a)} = -(b - a)
   \]
   Therefore, the equation of the line \(PQ\) is:
   \[
   y - a^2 = -(b - a)(x - a)
   \]
   Simplifying, we get:
   \[
   y = -(b - a)x + a^2 + a(b - a)
   \]
   \[
   y = -(b - a)x + ab
   \]

2. **Area \(S_1\) Bounded by the Line \(PQ\) and the Parabola:**
   The area \(S_1\) is the area between the parabola \(y = x^2\) and the line \(y = -(b - a)x + ab\) from \(x = -b\) to \(x = a\). The integrand for this area is:
   \[
   S_1 = \int_{-b}^{a} \left[(-(b - a)x + ab) - x^2\right] \, dx
   \]
   Simplifying the integrand:
   \[
   S_1 = \int_{-b}^{a} \left[-(b - a)x + ab - x^2\right] \, dx
   \]
   We can split this into three separate integrals:
   \[
   S_1 = -\int_{-b}^{a} (b - a)x \, dx + \int_{-b}^{a} ab \, dx - \int_{-b}^{a} x^2 \, dx
   \]

3. **Evaluating the Integrals:**
   \[
   -\int_{-b}^{a} (b - a)x \, dx = -(b - a) \left[ \frac{x^2}{2} \right]_{-b}^{a} = -(b - a) \left( \frac{a^2}{2} - \frac{b^2}{2} \right) = -\frac{(b - a)(a^2 - b^2)}{2}
   \]
   \[
   \int_{-b}^{a} ab \, dx = ab \left[ x \right]_{-b}^{a} = ab(a - (-b)) = ab(a + b)
   \]
   \[
   -\int_{-b}^{a} x^2 \, dx = -\left[ \frac{x^3}{3} \right]_{-b}^{a} = -\left( \frac{a^3}{3} - \frac{(-b)^3}{3} \right) = -\frac{a^3}{3} + \frac{b^3}{3}
   \]

4. **Combining the Results:**
   \[
   S_1 = -\frac{(b - a)(a^2 - b^2)}{2} + ab(a + b) - \frac{a^3}{3} + \frac{b^3}{3}
   \]
   Simplifying further:
   \[
   S_1 = -\frac{(b - a)(a + b)(a - b)}{2} + ab(a + b) - \frac{a^3}{3} + \frac{b^3}{3}
   \]
   \[
   S_1 = -\frac{(b^2 - a^2)(a - b)}{2} + ab(a + b) - \frac{a^3}{3} + \frac{b^3}{3}
   \]
   \[
   S_1 = -\frac{(b^3 - a^3)}{2} + ab(a + b) - \frac{a^3}{3} + \frac{b^3}{3}
   \]
   \[
   S_1 = \frac{1}{6}(a + b)^3
   \]

5. **Area \(S_2\) of Triangle \(OPQ\):**
   The area \(S_2\) of the triangle \(OPQ\) can be found using the determinant formula for the area of a triangle with vertices \((x_1, y_1)\), \((x_2, y_2)\), and \((x_3, y_3)\):
   \[
   S_2 = \frac{1}{2} \left| x_1(y_2 - y_3) + x_2(y_3 - y_1) + x_3(y_1 - y_2) \right|
   \]
   Substituting \(O(0, 0)\), \(P(a, a^2)\), and \(Q(-b, b^2)\):
   \[
   S_2 = \frac{1}{2} \left| 0(a^2 - b^2) + a(b^2 - 0) + (-b)(0 - a^2) \right|
   \]
   \[
   S_2 = \frac{1}{2} \left| ab^2 + ba^2 \right| = \frac{1}{2} \left| ab(a + b) \right| = \frac{1}{2} ab(a + b)
   \]

6. **Ratio \(\frac{S_1}{S_2}\):**
   \[
   \frac{S_1}{S_2} = \frac{\frac{1}{6}(a + b)^3}{\frac{1}{2} ab(a + b)} = \frac{(a + b)^2}{3ab}
   \]

7. **Minimizing the Ratio:**
   To find the minimum value of \(\frac{(a + b)^2}{3ab}\), we use the AM-GM inequality:
   \[
   a + b \geq 2\sqrt{ab}
   \]
   Squaring both sides:
   \[
   (a + b)^2 \geq 4ab
   \]
   Therefore:
   \[
   \frac{(a + b)^2}{3ab} \geq \frac{4ab}{3ab} = \frac{4}{3}
   \]
   The minimum value is achieved when \(a = b\), giving:
   \[
   \frac{(a + b)^2}{3ab} = \frac{(2a)^2}{3a^2} = \frac{4a^2}{3a^2} = \frac{4}{3}
   \]

The final answer is \( \boxed{ \frac{4}{3} } \)