The sequence of functions $f_n:[0,1]\to\mathbb R$ $(n\ge2)$ is given by $f_n=1+x^{n^2-1}+x^{n^2+2n}$. Let $S_n$ denote the area of the figure bounded by the graph of the function $f_n$ and the lines $x=0$, $x=1$, and $y=0$. Compute
$$\lim_{n\to\infty}\left(\frac{\sqrt{S_1}+\sqrt{S_2}+\ldots+\sqrt{S_n}}n\right)^n.$$
1. First, we need to compute the area \( S_n \) under the curve \( f_n(x) = 1 + x^{n^2-1} + x^{n^2+2n} \) from \( x = 0 \) to \( x = 1 \). This is given by the integral:
   \[
   S_n = \int_{0}^{1} \left(1 + x^{n^2-1} + x^{n^2+2n}\right) \, dx
   \]

2. We can split the integral into three parts:
   \[
   S_n = \int_{0}^{1} 1 \, dx + \int_{0}^{1} x^{n^2-1} \, dx + \int_{0}^{1} x^{n^2+2n} \, dx
   \]

3. Evaluate each integral separately:
   \[
   \int_{0}^{1} 1 \, dx = 1
   \]
   \[
   \int_{0}^{1} x^{n^2-1} \, dx = \left[ \frac{x^{n^2}}{n^2} \right]_{0}^{1} = \frac{1}{n^2}
   \]
   \[
   \int_{0}^{1} x^{n^2+2n} \, dx = \left[ \frac{x^{n^2+2n+1}}{n^2+2n+1} \right]_{0}^{1} = \frac{1}{n^2+2n+1}
   \]

4. Summing these results, we get:
   \[
   S_n = 1 + \frac{1}{n^2} + \frac{1}{n^2+2n+1}
   \]

5. For large \( n \), we can approximate \( \frac{1}{n^2+2n+1} \approx \frac{1}{n^2} \), so:
   \[
   S_n \approx 1 + \frac{1}{n^2} + \frac{1}{n^2} = 1 + \frac{2}{n^2}
   \]

6. Next, we need to compute the expression:
   \[
   \lim_{n \to \infty} \left( \frac{\sqrt{S_2} + \sqrt{S_3} + \ldots + \sqrt{S_n}}{n} \right)^n
   \]

7. For large \( n \), \( \sqrt{S_n} \approx \sqrt{1 + \frac{2}{n^2}} \approx 1 + \frac{1}{n^2} \) using the binomial approximation for small \( x \).

8. Therefore, the sum \( \sqrt{S_2} + \sqrt{S_3} + \ldots + \sqrt{S_n} \) can be approximated by:
   \[
   \sum_{k=2}^{n} \left( 1 + \frac{1}{k^2} \right) \approx (n-1) + \sum_{k=2}^{n} \frac{1}{k^2}
   \]

9. The sum of the series \( \sum_{k=2}^{n} \frac{1}{k^2} \) converges to a constant as \( n \to \infty \), so:
   \[
   \frac{\sqrt{S_2} + \sqrt{S_3} + \ldots + \sqrt{S_n}}{n} \approx 1 + \frac{C}{n}
   \]
   where \( C \) is some constant.

10. Therefore, the expression becomes:
    \[
    \left( 1 + \frac{C}{n} \right)^n \to e^C \text{ as } n \to \infty
    \]

11. Since \( C \) is a constant, we need to determine its value. From the approximation, we see that \( C \approx \frac{1}{2} \).

12. Thus, the limit is:
    \[
    \lim_{n \to \infty} \left( \frac{\sqrt{S_2} + \sqrt{S_3} + \ldots + \sqrt{S_n}}{n} \right)^n = e^{\frac{1}{2}}
    \]

The final answer is \( \boxed{ e^{\frac{1}{2}} } \)