For positive integers $k = 1, 2, 3, \ldots, n$, let $S_k$ denotes the area of $\triangle AOB_k$ (where 'O' is origin) such that $\angle AOB_k = \frac{k\pi}{2n}$, $OA = 1$ and $OB_k = k$. If the value of $\lim_{n \to \infty} \frac{1}{n^2} \sum_{k=1}^n S_k = \frac{a}{\pi^2}$, then 'a' is equal to
Let $f(x)$ be a continuous function with continuous first derivative on $(a, b)$, where $b > a$, and let $\lim_{x \to a^+} f(x) = \infty$, $\lim_{x \to b^-} f(x) = -\infty$ and $f'(x) + f^2(x) \geq -1$, for all $x$ in $(a, b)$, if the minimum value of $(b-a)$ equals to $k$ then $k$ is ____.