Let \( f(x) \) be a continuous function \( \forall\, x \in \mathbb{R} \) such that \[ \lim_{x \to \pi/4} \frac{\displaystyle\int_{2}^{\sec^2 x} f(t)\, dt}{x^2 - \dfrac{\pi^2}{16}} = \frac{k}{\pi} f(a) \] where \( a, k \in \mathbb{N} \), then the value of \( k^a \) is equal to: