Find $\lim\limits_{n \to \infty}\bigg ( \lim\limits_{x \to 0}(1 + \tan^2(x) + \tan^2(2x) + ... + \tan^2(nx))^\frac{1}{n^3x^2} \bigg )$.
Evaluate $ \lim_{n\to \infty} ( \lim_{x\to0} (1+\tan^2(x)+\tan^2(2x)+ \cdots + \tan^2(nx)))^{\frac{1}{n^3x^2}} $