Let $f_n\ (n=1,\ 2,\ \cdots)$ be a linear transformation expressed by a matrix $\left(
\begin{array}{cc}
1-n & 1  \\
-n(n+1) & n+2
\end{array}
\right)$ on the $xy$ plane. Answer the following questions:

(1) Prove that there exists 2 lines passing through the origin $O(0,\ 0)$ such that  all points of the lines are mapped to the same lines, then find the equation of the lines.

(2) Find the area $S_n$ of the figure enclosed by the lines obtained in (1) and the curve $y=x^2$.

(3) Find $\sum_{n=1}^{\infty} \frac{1}{S_n-\frac 16}.$

[i]2011 Tokyo Institute of Technlogy entrance exam, Problem 1[/i]
### (1) Prove that there exist 2 lines passing through the origin \( O(0, 0) \) such that all points of the lines are mapped to the same lines, then find the equation of the lines.

Given the linear transformation matrix:
\[
A_n = \begin{pmatrix}
1-n & 1 \\
-n(n+1) & n+2
\end{pmatrix}
\]

We need to find lines passing through the origin that are invariant under this transformation. Let the equation of the line be \( y = mx \). For the line to be invariant, the direction vector \( \begin{pmatrix} 1 \\ m \end{pmatrix} \) must be an eigenvector of \( A_n \).

Thus, we need:
\[
A_n \begin{pmatrix} 1 \\ m \end{pmatrix} = \lambda \begin{pmatrix} 1 \\ m \end{pmatrix}
\]

This gives us the system of equations:
\[
\begin{pmatrix}
1-n & 1 \\
-n(n+1) & n+2
\end{pmatrix}
\begin{pmatrix}
1 \\
m
\end{pmatrix}
=
\begin{pmatrix}
(1-n) + m \\
-n(n+1) + (n+2)m
\end{pmatrix}
=
\lambda \begin{pmatrix}
1 \\
m
\end{pmatrix}
\]

Equating components, we get:
\[
1 - n + m = \lambda
\]
\[
-n(n+1) + (n+2)m = \lambda m
\]

From the first equation:
\[
\lambda = 1 - n + m
\]

Substitute \(\lambda\) into the second equation:
\[
-n(n+1) + (n+2)m = (1 - n + m)m
\]

Simplify:
\[
-n(n+1) + (n+2)m = m - nm + m^2
\]

Rearrange terms:
\[
m^2 - (2n+1)m + n(n+1) = 0
\]

This is a quadratic equation in \(m\). Solving for \(m\):
\[
m = \frac{(2n+1) \pm \sqrt{(2n+1)^2 - 4n(n+1)}}{2}
\]

Simplify the discriminant:
\[
(2n+1)^2 - 4n(n+1) = 4n^2 + 4n + 1 - 4n^2 - 4n = 1
\]

Thus:
\[
m = \frac{(2n+1) \pm 1}{2}
\]

So, the solutions are:
\[
m = n \quad \text{and} \quad m = n+1
\]

Therefore, the invariant lines are:
\[
\boxed{y = nx \quad \text{and} \quad y = (n+1)x}
\]

### (2) Find the area \( S_n \) of the figure enclosed by the lines obtained in (1) and the curve \( y = x^2 \).

The lines are \( y = nx \) and \( y = (n+1)x \). We need to find the area between these lines and the curve \( y = x^2 \).

First, find the points of intersection of \( y = x^2 \) with \( y = nx \) and \( y = (n+1)x \):
\[
x^2 = nx \implies x = 0 \text{ or } x = n
\]
\[
x^2 = (n+1)x \implies x = 0 \text{ or } x = n+1
\]

The area \( T_n \) between \( y = x^2 \) and \( y = nx \) from \( x = 0 \) to \( x = n \) is:
\[
T_n = \int_0^n (nx - x^2) \, dx
\]

Calculate the integral:
\[
T_n = \int_0^n nx \, dx - \int_0^n x^2 \, dx = \left[ \frac{nx^2}{2} \right]_0^n - \left[ \frac{x^3}{3} \right]_0^n = \frac{n^3}{2} - \frac{n^3}{3} = \frac{n^3}{6}
\]

Similarly, the area \( T_{n+1} \) between \( y = x^2 \) and \( y = (n+1)x \) from \( x = 0 \) to \( x = n+1 \) is:
\[
T_{n+1} = \int_0^{n+1} ((n+1)x - x^2) \, dx
\]

Calculate the integral:
\[
T_{n+1} = \int_0^{n+1} (n+1)x \, dx - \int_0^{n+1} x^2 \, dx = \left[ \frac{(n+1)x^2}{2} \right]_0^{n+1} - \left[ \frac{x^3}{3} \right]_0^{n+1} = \frac{(n+1)^3}{2} - \frac{(n+1)^3}{3} = \frac{(n+1)^3}{6}
\]

The area \( S_n \) between the two lines and the curve is:
\[
S_n = T_{n+1} - T_n = \frac{(n+1)^3}{6} - \frac{n^3}{6} = \frac{(n+1)^3 - n^3}{6}
\]

Simplify:
\[
(n+1)^3 - n^3 = (n^3 + 3n^2 + 3n + 1) - n^3 = 3n^2 + 3n + 1
\]

Thus:
\[
S_n = \frac{3n^2 + 3n + 1}{6} = \boxed{\frac{1}{6}(3n^2 + 3n + 1)}
\]

### (3) Find \( \sum_{n=1}^{\infty} \frac{1}{S_n - \frac{1}{6}} \).

From part (2), we have:
\[
S_n = \frac{1}{6}(3n^2 + 3n + 1)
\]

Thus:
\[
S_n - \frac{1}{6} = \frac{1}{6}(3n^2 + 3n + 1) - \frac{1}{6} = \frac{1}{6}(3n^2 + 3n) = \frac{1}{2}n(n+1)
\]

Therefore:
\[
\frac{1}{S_n - \frac{1}{6}} = \frac{2}{n(n+1)}
\]

We need to find:
\[
\sum_{n=1}^{\infty} \frac{2}{n(n+1)}
\]

Using partial fractions:
\[
\frac{2}{n(n+1)} = 2 \left( \frac{1}{n} - \frac{1}{n+1} \right)
\]

Thus:
\[
\sum_{n=1}^{\infty} \frac{2}{n(n+1)} = 2 \sum_{n=1}^{\infty} \left( \frac{1}{n} - \frac{1}{n+1} \right)
\]

This is a telescoping series:
\[
2 \left( 1 - \frac{1}{2} + \frac{1}{2} - \frac{1}{3} + \frac{1}{3} - \frac{1}{4} + \cdots \right) = 2
\]

Thus:
\[
\sum_{n=1}^{\infty} \frac{1}{S_n - \frac{1}{6}} = \boxed{2}
\]