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}$.